secrec If you were running the factory installed operating system, then you couldn't be sure, but if you're running GrapheneOS, then all the google spyware is not there, so definitely not stealing your fingerprints.
A lot is known about the Android fingerprint system. According to the design, fingerprint pictures are not stored, only profiles, and they are secured inside the trusted key/DRM system. If the OS doesn't have a fingerprint image, it can't steal it.
Of course it's possible that Google's representations about how the system works are inaccurate. But people have inspected Android phones to break into them through the fingerprint system (source), and they did not report that the implementation diverged from the spec in that way (sadly, they did not test their attack on Pixel devices).