A lot of android apps rely on GCM (google’s notification servers), I assume Whatsapp is one. Signal does too, but I think it can use its own websockets when gcm is not available, same for telegram. Whatsapp probably not.
Some ROMs with no google services restore the gcm funciinality via microg. Graphene os, as far as i know, does not have microg.
Perhaps you didn’t read the part where it says it is opt-in…? Or maybe made up the part where it says that the classic local backup mechanism will be removed?