This fix will be part of the next release of the ArcGIS Viewer for Flex.
One work-around that might help you in the meantime (if you're not OK with using fewer fields, nor with compiling the source code), is to use the <description> tag.
From the documentation on Pop-up configuration files:
"By default the pop-up window shows the visible fields in a key-value styled table. Alternatively, you can use the <description> tag to present it in a different manner, for example in a sentence or paragraph structure"
http://help.arcgis.com/en/webapps/flexviewer/help/#/Pop_up_configuration_files/01m30000002q000000/
<configuration>
<title>{NAME}</title>
<description>
<![CDATA[<b>Hurricane {NAME}</b> was a <i>category {CAT}</i> hurricane that occurred in {BASIN} on {TRACK_DATE}. It had a wind speed of {WIND_KTS} knots.]]>
</description>
<fields>
<field name="NAME"/>
<field name="WIND_KTS"/>
<field name="CAT"/>
<field name="BASIN"/>
<field name="TRACK_DATE">
<format dateformat="shortDateShortTime"/>
</field>
</fields>
</configuration><configuration>
<title>{OwnershipRecords.OWNER}</title>
<description>
<![CDATA [[Parcel ID: {OwnershipRecords.PID} <br>
Owner: {OwnershipRecords.OWNER} <br>
Mailing Address: {OwnershipRecords.MAILADDRES} <br>]>
</description>
<fields>
<field name="OwnershipRecords.PID" alias="Parcel ID" visible="true"/>
<field name="OwnershipRecords.OWNER" alias="Owner" visible="true"/>
<field name="OwnershipRecords.MAILADDRES" alias="Mailing Address" visible="true"/>
<field name="OwnershipRecords.MAILCITY" alias="Mailing City" visible="true"/>
<field name="OwnershipRecords.MAILST" alias="Mailing State" visible="true"/>
<field name="OwnershipRecords.MAILZIP" alias="Mailing Zip Code" visible="true"/>
<field name="OwnershipRecords.LOT" alias="Lot" visible="true"/>
<field name="OwnershipRecords.BLK" alias="Block" visible="true"/>
<field name="OwnershipRecords.TRACT" alias="Legal Description" visible="true"/>
<field name="Subdiv code table.Subdivisio" alias="Subdivision" visible="true"/>
<field name="OwnershipRecords.SEC" alias="Section" visible="true"/>
<field name="OwnershipRecords.TWP" alias="Township" visible="true"/>
<field name="OwnershipRecords.RNG" alias="Range" visible="true"/>
<field name="OwnershipRecords.CALCACRES" alias="Acres" visible="true"/>
<field name="OwnershipRecords.PROPNUM" alias="Number" visible="true"/>
<field name="OwnershipRecords.PROPDIR" alias="Direction" visible="true"/>
<field name="OwnershipRecords.PROPST" alias="Street" visible="true"/>
<field name="OwnershipRecords.LOTWIDTH" alias="Lot Width" visible="true"/>
<field name="OwnershipRecords.LOTDEPTH" alias="Lot Depth" visible="true"/>
</fields>
<showattachments>true</showattachments>
</configuration>
<configuration>
<title>{NAME}</title>
<description>
<![CDATA[<b>Hurricane {NAME}</b> was a <i>category {CAT}</i> hurricane that occurred in {BASIN} on {TRACK_DATE}. It had a wind speed of {WIND_KTS} knots.]]>
</description>
<fields>
<field name="NAME"/>
<field name="WIND_KTS"/>
<field name="CAT"/>
<field name="BASIN"/>
<field name="TRACK_DATE">
<format dateformat="shortDateShortTime"/>
</field>
</fields>
</configuration>Use either <description> or <fields> for the main content.
<configuration>
<title>{OwnershipRecords.OWNER}</title>
<description>
<![CDATA [[Parcel ID: {OwnershipRecords.PID} <br>
Owner: {OwnershipRecords.OWNER} <br>
Mailing Address: {OwnershipRecords.MAILADDRES} <br>
Continue to add all your fields here: {OwnershipRecords.???} <br>]]>
</description>
<fields>
<field name="OwnershipRecords.PID" />
<field name="OwnershipRecords.OWNER" />
<field name="OwnershipRecords.MAILADDRES" />
<field name="OwnershipRecords.MAILCITY" />
<field name="OwnershipRecords.MAILST" />
<field name="OwnershipRecords.MAILZIP" />
<field name="OwnershipRecords.LOT" />
<field name="OwnershipRecords.BLK" />
<field name="OwnershipRecords.TRACT" />
<field name="Subdiv code table.Subdivisio" />
<field name="OwnershipRecords.SEC" />
<field name="OwnershipRecords.TWP" />
<field name="OwnershipRecords.RNG" />
<field name="OwnershipRecords.CALCACRES" />
<field name="OwnershipRecords.PROPNUM" />
<field name="OwnershipRecords.PROPDIR" />
<field name="OwnershipRecords.PROPST" />
<field name="OwnershipRecords.LOTWIDTH" />
<field name="OwnershipRecords.LOTDEPTH" />
</fields>
<showattachments>true</showattachments>
</configuration>
<?xml version="1.0" ?> <configuration> <title>{OwnershipRecords.OWNER}</title> <description> <![CDATA [Parcel ID: {OwnershipRecords.PID} <br> Owner: {OwnershipRecords.OWNER} <br> Mailing Address: {OwnershipRecords.MAILADDRES} <br> Continue to add all your fields here: {OwnershipRecords.???} <br>]]> </description> <fields> <field name="OwnershipRecords.PID" /> <field name="OwnershipRecords.OWNER" /> <field name="OwnershipRecords.MAILADDRES" /> <field name="OwnershipRecords.MAILCITY" /> <field name="OwnershipRecords.MAILST" /> <field name="OwnershipRecords.MAILZIP" /> <field name="OwnershipRecords.LOT" /> <field name="OwnershipRecords.BLK" /> <field name="OwnershipRecords.TRACT" /> <field name="Subdiv code table.Subdivisio" /> <!-- I am wondering about the above field name is that correct? --> <field name="OwnershipRecords.SEC" /> <field name="OwnershipRecords.TWP" /> <field name="OwnershipRecords.RNG" /> <field name="OwnershipRecords.CALCACRES" /> <field name="OwnershipRecords.PROPNUM" /> <field name="OwnershipRecords.PROPDIR" /> <field name="OwnershipRecords.PROPST" /> <field name="OwnershipRecords.LOTWIDTH" /> <field name="OwnershipRecords.LOTDEPTH" /> </fields> <showattachments>true</showattachments> </configuration>
<configuration>
<title>{OwnershipRecords.OWNER}</title>
<description>
<![CDATA [Parcel ID: {OwnershipRecords.PID} <br>
Owner: {OwnershipRecords.OWNER} <br>
Mailing Address: {OwnershipRecords.MAILADDRES} ]]
</description>
<fields>
<field name="OwnershipRecords.PID"/>
<field name="OwnershipRecords.OWNER"/>
<field name="OwnershipRecords.MAILADDRES"/>
<field name="OwnershipRecords.MAILCITY"/>
<field name="OwnershipRecords.MAILST"/>
<field name="OwnershipRecords.MAILZIP"/>
<field name="OwnershipRecords.LOT"/>
<field name="OwnershipRecords.BLK"/>
<field name="OwnershipRecords.TRACT"/>
<field name="Subdiv code table.Subdivisio"/>
<field name="OwnershipRecords.SEC"/>
<field name="OwnershipRecords.TWP"/>
<field name="OwnershipRecords.RNG"/>
<field name="OwnershipRecords.CALCACRES"/>
<field name="OwnershipRecords.PROPNUM"/>
<field name="OwnershipRecords.PROPDIR"/>
<field name="OwnershipRecords.PROPST"/>
<field name="OwnershipRecords.LOTWIDTH"/>
<field name="OwnershipRecords.LOTDEPTH"/>
</fields>
<showattachments>true</showattachments>
</configuration>
<?xml version="1.0" ?> <configuration> <title>{OwnershipRecords.OWNER}</title> <description> <![CDATA [Parcel ID: {OwnershipRecords.PID} <br> Owner: {OwnershipRecords.OWNER} <br> Mailing Address: {OwnershipRecords.MAILADDRES} ]]> </description> <fields> <field name="OwnershipRecords.PID"/> <field name="OwnershipRecords.OWNER"/> <field name="OwnershipRecords.MAILADDRES"/> <field name="OwnershipRecords.MAILCITY"/> <field name="OwnershipRecords.MAILST"/> <field name="OwnershipRecords.MAILZIP"/> <field name="OwnershipRecords.LOT"/> <field name="OwnershipRecords.BLK"/> <field name="OwnershipRecords.TRACT"/> <field name="Subdiv code table.Subdivisio"/> <field name="OwnershipRecords.SEC"/> <field name="OwnershipRecords.TWP"/> <field name="OwnershipRecords.RNG"/> <field name="OwnershipRecords.CALCACRES"/> <field name="OwnershipRecords.PROPNUM"/> <field name="OwnershipRecords.PROPDIR"/> <field name="OwnershipRecords.PROPST"/> <field name="OwnershipRecords.LOTWIDTH"/> <field name="OwnershipRecords.LOTDEPTH"/> </fields> <showattachments>true</showattachments> </configuration>