We can with some more information, do you have third party app/services syncing contacts? Are you removing them from these services elsewhere which could result in them being removed from your device on sync?
Contacts don't just get deleted on GrapheneOS. It's not the OS that will be doing this. I've moved across 6 generations of Pixels with backups and restores and not even lost contacts through any of that.
We need to know more of your config/setup, have you got developer options enabled, have you made any changes, have you enabled USB debugging, used adb and/or anything like Shizuku etc