TiggyTheTerrible all you can do is to contact the devs of that app and explain why they should put GrapheneOS on a whitelist.
Unfortunely, GrapheneOS is not certified by Google and some devs only allow their apps to run on certified devices.
This certification is a also a big joke.
From Google " The Android team at Google certifies these devices to ensure they are secure and ready to run apps from Google and the Play Store. "
GrapheneOS who is hardened and runs latest android release and security patch is automatically considered as insecure but an old android device is safe and secure to run even if was last updated 5 years ago because it's certified. Something is wrong right ?
I have no regrets to have GrapheneOS as my daily driver. I have more to lose by running stock OS than GrapheneOS.