google pixel 4a – 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 too)

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 (I am not talking about the requirement after a re-boot).

Pixel 4a, Android 11