mmobder
Well I mentioned that I couldn't determine a pattern, so I didn't know what useful details I can give you.
Apps that have access to contacts:
Messaging (GOS), Phone (GOS), PM Mail, settings (GOS), and f'ing WhatsApp
so not so many variables
No synching contacts with anything
Also a second person having the same problem on Pixel9pro
It happened multiple times, it's not a one of, as mentioned in OP I had to re save my GF number, in the meantime I had to save it again (holding strong since a week or so now 👍)
It isn't easy to see a pattern because you don't get any notification/sign when a contact gets deleted (obviously)
You just notice if when you look fore someone's contact, when you see "unknown numbers" in your "recent calls", when you receive a call from a "unknown" number but then it's someone you talk with regularly or in chats when you see someone's number displayed instead of their name
So ok, maybe I f'd up (would be interesting to know where), but there's a chance that something else is wrong