It isn't due to the Play Integrity API. That shows a notification on GrapheneOS with a toggle for blocking it to work around it for services not enforcing providing a result. If that was the issue, the original poster would have known from the notification. The issue ended up being PayPal shipping incorrect anti-tampering code incompatible with secure spawning. The original poster figured that out and got it working by disabling the per-app secure spawning toggle.