Hi @TravisSizemore thanks for the post.
At the recent ArcGIS Online update there was a new capability added to make attribute fields non-editable from the UI. Since you mentioned that in the service definition, the field is still editable I would presume it also shows as editable on the Data > fields > <field> tab. On this tab, does a small pencil icon appear in the Editable section to toggle editing on/off?
I am not seeing any issues on my end with editing fields that have domains in the attribute table, but we would like to investigate this further through Esri Support.
At your convenience, would you be able to send me your customer number in a direct message or email to email@example.com so I can get a case started for you?