Hi subacoustic, I've just set up my Triodos UK banking app on my GrapheneOS Pixel 6a.
I found that I was NOT able to do it using the recommended option of scanning a QR code from the Triodos app on my old non-Graphene phone (the QR code appeared to be simply undetected when I pointed my new Pixel 6a at it). And maybe this is what you found too?
But when I used my Triodos Digipass instead that allowed me to log in and fully set up the app. As far as I can tell so far it's now working fine.