In my experience the fingerprint sensor that comes with the Pixel Tablet can be a bit unreliable sometimes. This is not something GOS-exclusive, it also happens with the stock OS.
I suggest you try to delete all stored fingerprints and record them again. Another trick would be to record the same finger twice (this seems to help to some extent). Also when trying to unlock your tablet, pay attention to where you position your finger and how you do it (the amount of pressure seems to be a factor on the success rate).
If none of the above works, we could start to consider as an hypothesis a hardware issue.