If it's a GrapheneOS installed using the official guide to a Pixel, I trust it with my fingerprint.

This article has some good info about how fingerprints are handled by an android phone:

https://infinum.com/blog/android-fingerprint-security

Nothing is impossible, but having a back-door to steal your fingerprint would be a lot more difficult than following you and collecting your prints off something that you touched. Alternatively, your fingerprints can be extracted from a good-resolution photograph of your hands.

So... Fingerprints are not super secure, but your phone stealing them is not so likely.