I am using ArcGIS 10.3.1 on my local and 10.4 on the server.
I constructed a model that at one point appends 2 layers and then dissolves the data. The append accurately appends data from both layers but the dissolve only contains data from one layer. I have this same process in 4 other models and it works for those but not this one. This process does work when I run it in modelbuilder. My python script was exported directly from the working module. I have compared the script against the working scripts and can find no difference in the procedures.
For Example (Cliff Notes Version):
LayerNew: New data that was just buffered
LayerOld: Old data buffered that needs to be updated
Append LayerNew with LayerOld. Outcome is LayerOld containing data from LayerOld and LayerNew.
Dissolve LayerOld. Outcome is only data from LayerNew.
# Process: Buffer
arcpy.Buffer_analysis(GeoRecsNew_AllBuff_NS__2_, ServerConnection_GeoRecsNewBuff_NoNulls_Sym_NS, "1 Miles", "FULL", "ROUND", "LIST", "FileID;WorkStatus;Subs_new;Subs_Current;Subs_Finished;DateAdded_month;NodeDissolved;LotCount_Finished", "PLANAR")
# Process: Append
arcpy.Append_management("'Database Connections\\ServerConnection\\ServerFolder\\ServerConnection.GeoRecsNewBuff_NoNulls_Sym_NS'", ServerConnection_GeoRecsOldBuff_NoNulls_Sym_NS, "TEST", "", "")
print ("append buffers")
# Process: Dissolve
arcpy.Dissolve_management(ServerConnection_GeoRecsOldBuff_NoNulls_Sym_NS__2_, ServerConnection_GeoRecsOldBuff_SymDissolve_NS, "FileID;WorkStatus;Subs_new;Subs_Current;Subs_Finished;DateAdded_month;NodeDissolved;LotCount_Finished", "", "MULTI_PART", "DISSOLVE_LINES")
print ("dissolve buffers")