Similar behaviour here:
from time to time, the phone just stays black for up to 20 or 30 seconds and doesn’t respond to any input (the power button being pushed or the fingerprint sensor being touched).
Last time the alarm kept ringing all the time while the phone stayed unresponsive (could only turn it off after everything got back to normal again).
I’m pretty sure this behaviour didn’t show from the beginning - it has probably started about two weeks ago.
Here’s another thread describing a similar behaviour: Fairphone 3 takes a long time to react after standby (up to 5 seconds until screen turns on)