Mojave, but it happened many times before in previous versions, and I think also in PM5. I was too lazy to report it earlier, but a couple of days ago it got me in serious trouble.
My OS is in Dutch, and 'bas' is a common word in Dutch (means 'bass'), so that may be the culprit.
UPDATE!
I found the solution: If you open Edit / Spelling / Show Spelling and Grammar you can add words to the thesaurus by typing the word and click the learn button. So I just learned Spelling to accept fbas, and it seems to work everywhere now.
But:
The reason I had not found this option before is that, when you have the Metadata (IPTC) screen open and you open the Show Spelling and Grammar dialog, the latter opens BEHIND the Metadata (IPTC) screen. This gives the user the impression you can't use it when the Metadata (IPTC) screen is active. My advice: the latest pop-up screen should always be on top.