Unfortunately, I have this exact issue too. I installed the latest build of GOS yesterday, expecting WhatsApp to work after some freak accident on the stock rom (you need the official whatsapp app to log in, but i got it on the stock rom no I am not kidding), because I already registered my new account with my new number, however, it still didnt work on GOS. I had to wipe it off after much deliberation (I was a happy GOS user for many, many, many months prior on a different phone, when I got a new one I didn't have time to put GOS on it), and I installed the android 16 beta build directly from the factory image, which still didn't work. I had to flash stock android 15 for it to work again.
I suspect it has to do with both play integrity but also if your bootloader is unlocked in the first place or if you have OEM unlocking on, two settings/states I both set to off after I flashed stock android 15. I think stable android may also be a hard requirement (??). Either way, I have concluded that it is impossible for me to use GOS anymore just because stupid meta wants me to be spied on by both themselves and meta, and I can't do anything about it because I have many services on WhatsApp I need to have (contact with organizations, second hand sellers and even the government).