If I understand you correctly, your screen just stays black for some time (and the phone does not respond to power-button being pressed) - only after some time, it works just fine again?
There’s two more threads in the forum covering such issues:
- Switch on my FP3 takes too much time
- Fairphone 3 takes a long time to react after standby (up to 5 seconds until screen turns on)
Similar behaviour here - still trying to narrow it down when it occurs - last time it occured when connecting to the car’s bluetooth (but it doesn’t always occur then).