Just updated to V21 following Planet Computer instructions after fingerprints failed to work on V20.
I noticed that Android Security Update is now 5 July 2019. I thought V20 had a more recent security update, but I can't remember now.
Fingerprint error seems to be fixed, but every time I open the Cosmo after it has gone into sleep mode (closing cover) for a few minutes, I find that the system has either shut down or rebooted.