ArcGIS Pro 2.3.2 not saving domain changes for subtype fields, even though I am the owner of the database and the domains are in the database.
I have the same issue whether I am in FGDB or SQL Enterprise DB.
Example (FGDB) -
When I try to add the existing domain 'UoM' (units of measure) to the UoM field for piping devices, I am able to select the domain and it stays for all of the subtype options for that field. (Attachment Domain1)
As soon as I save, the data disappears and I get the red bar in front of the first subtype and the error code 999999 (Attachment Domain2)
This is repeatable - I have rebooted, saved, closed the app, anything I can think of to clear the 'cache', but to no avail!
Not that it helps you but this is one of the major reasons why we abandoned subtypes a number of years ago.
In ArcMap you sometimes had to individually assign the domain to each subtype.
Pro is set up much nicer for managing subtypes.
Issues I can speculate on is you may need to unregister as versioned. Stop all services running on the feature. Remove any Relationships that may exist