The non-perfect grip explains some of the failed readings, but not all. I can’t tell how often it happens (maybe once or twice a week for me?), but I have to restart the fingerprint sensor daemon every now and then to be able to unlock the 10 III without typing. The last time I did it was last night and the restart fixed the issue immediately.
Apparently it happens quite often, since there was a suggestion regarding Google assistant button and the fingerprint sensor daemon.