I'm working with an electric UNv7 in an MGDB using Pro 3.5.5. I am attempting to add a summary attribute to my subnetwork definition that provides a count of how many different types of service locations are on each subnetwork (i.e., residential vs commercial). However that differentiation is at the asset type level and when i attempt to create a new summary attribute using Asset Type I get "Error 000800: The value is not a member of <Unknown>.

Comparing using the Asset Group vs Asset Type as the attribute, i notice that the filter value changes from free text to a drop down, and with asset type the drop down only includes an Unknown value. Is this potentially the tool having an issue aggregating the possible asset types across all the asset groups?
It reminds me of when we used to notice similar behavior in the Select by Attribute tool when viewing non-subtype group layers; Unknown was the only option that would appear for asset type because it was the only one every group had in common.
@RobertKrisher @MikeMillerGIS