Another logical thing to try is to review the FGDB Database properties to make sure it still see it there. It would be goo to check the domain Name, and coded values. Then look at the field properties and make sure it is still assigned correctly. There is nothing in what you said that would indicate that there is a problem with either, but never hurts to check. Never hurts to check the basics, just to rule it out.
Just fyi:
I have a script that runs periodically that recreates many of my layers and reassigns all the domains. This may be overkill, but I figure that way if I change something, i force it to be updated. I have many fairly complicated domains in my master.
I have all my domain files stored in a FGDB, then run thru a few loops using the command
arcpy.TableToDomain_management