I have installed latest LingeageOS 22.1 (updated yesterday to A15) on my FP5 and have the same issue. This means it is not related to the Fairphone OS but the firmware itself.
Holding volume down first let the volume go down to zero and afterwards (hold it for ~15 sec) just force shuts down the FP5. Vol up is fine. I can also power up the phone afterwards but as my bootloader is still unlocked, I need to press the power button to continue. I don’t need this if I shut down the phone with the power button and press “shut down”. So it seems to be a power cut off or other special “emergency power down”. - Or a firmware bug. 