Conditions for triggering "pin required for additional security“

As in title, what are the conditions? (fingerprint isn't enough to unlock the device, one needs to enter PIN. I am not talking about the requirement after a re-boot) I can't seem to discern a pattern, on some days I get this security warning at least 3 times, and on some days never or maybe once, while the usage pattern seems to be similar. Pixel 4a, Android 11