Just yesterday I experienced this myself on Xpreria 10II with SFOS 4.2.0.21 (Verla). Even after rebooting fingerprint detection was not operational, and when I went to settings to check it out the fingerprints available the selection was empty.
However, when I entered my security code to add new ones it complained about “insufficient memory” or some such, but suddenly the fingerprints were again there and unlocking started working.
As bluetooth has been suspected, I have my phone connect/disconnect from the car handsfree every time I drive (pretty much daily).