Couple questions: what happens when you try a record where that field has a value? Also, what type of GDB is the data stored in? If in an enterprise gdb, does you user have read permissions to the domain?
Just to verify: In your table of contents, right click on the feature class and select Design; choose Domains and that will show you all the domains the the gdb. Click on fields and that will expose what Domain is assigned to what field in that feature class.
Same results if I try to edit a field attribute that has existing data. The data for this occurrence resides in and ESRI AGOL server as a feature service. The web map was added and opened in Pro. The features are part of the web map.
The TOC>Feature>Design>Fields shows the domain but it is ghosted as it is now in the AGOL and not desktop FGDB.
The same thing occurs when I open a new blank map and add the feature from the AGOL.
Not really a big deal as long as the Attributes Pane opens and the data can be edited there. Still some form of a glitch that should not happen.
It sounds very similar to what is reported here BUG-000129214: ArcGIS Pro 2.5 does not consistently display drop-do..
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