Isn't this more about PayPal disliking a rooted device rather than Graphene? I used to have all kinds of issues with financial apps when I was rooted, regardless of OS
It's been explained in this thread many times: not-rooted
GrapheneOS isn't rooted. The issue is their flawed anti-tampering code shipped a new bug breaking compatibility with secure spawning. We have a per-app toggle for secure spawning due to seeing this with other banking/financial/government apps and it works for PayPal's app as the original poster discovered.
Flawed root detection issues are kinda common to see when running Graphene, tbh I suspect this is just an accident - it very much is not the first time, nor the first for PayPal.