ArcGIS Collector Required Field Issue

We are hosting our feature service on Portal (version 10.6). The layer has 2 required fields (domain, non-nullable) and another field only used for symbology (domain, nullable). On the Portal web viewer, the edit window shows what we expect: the 2 required fields marked with an asterisk, and the other layer not required but still editable. However, when we view the map in Collector (version 20.2.1), all 3 fields show as required with an asterisk.

Has anyone seen anything similar or know of a work-around?