This issue is addressed in ArcGIS Pro 2.9.x and 3.2.
This issue is fixed and an updated Add-in is uploaded to Esri Community. Refer to: https://community.esri.com/t5/arcgis-utility-network-documents/select-by-association-tool-as-an-add-in-for-arcgis/ta-p/1200784 for more information.