Since it works on my Pixel 6 with the current 2025061000 GrapheneOS version, maybe it's related to this?
source: Did you prevent the Google Play services app from performing DCL via memory? It can erroneously detect this as a sign of "tampering" and downgrade your Play Integrity verdict from 'basic' to 'no integrity' because of this. If you did, try allowing it, then clear the cache of Google Play services and reboot.
I had to specifically allow DCL from memory for play services, for it to be active (default mode set to off). Didn't check my integrity status before, though.
I can just install the PayPal app from the Play Store, it's also instantly listed as a wallet option in the NFC settings (but I'm from germany, which as of now seems to be a factor for the latter). Didn't try logging in, as I use another NFC payment app and PayPal only online.
Tried using a german IP address. I don't know if Play Store's restrictions on device integrity can be set for certain areas? Shouldn't be the case.