As best I can deduce, there are three tables here, The layout's table, the portal's table and Health_session_ANIMALHEALTHTREATMENT_gSession_gTreatmentFiltered. Is that correct?
Any references to fields from a related table, unless using an aggregate function or referencing a summary field, only refer to the first related record.
assuming that Health_session_ANIMALHEALTHTREATMENT_gSession_gTreatmentFiltered is a related table occurrence of records separate from that specified for the portal and your layout, you need something like:
NotIsempty ( FilterValues ( List ( Health_session_ANIMALHEALTHTREATMENT_gSession_gTreatmentFiltered::_animalID_fk ) ; self ) )
Thank you, PhilModJunk, works like magic. Thanks, Bill