I manage several multi-user geodatabases (SQL, Versioned, 10.5.1) and have never had a problem with adding a new coded domain value to an existing domain set in any of the databases.
Currently I am having an issue with one of the databases not retaining the new coded domain.
Add a new coded value, it is there and everything appears fine.
Apply and refresh the database, value is still present.
Close and reopen ArcCatalog, open the database and the value is gone.
I found a similar post at How to add a new code value to existing domain? but the solution did not fit with the issues I am having.
There are no Subtypes, the domain is correctly connected to the field and all the existing codes work correctly.
I am owner and DBA for the database and this same layer happens to be in one of the other databases I manage. The domain applied correctly and with no problems in that database.