Hi everyone,
I had the same issue - my Fairphone 2 couldn’t shut down unless using the “flashlight” trick or going through the recovery mode.
And after I recently updated to Fairphone OS v19.02.1, the problem seemed to be fixed 
…well, until a few days ago. 
Now, although I’m indeed using the latest OS version, the same problem re-appeared: my phone automatically restarts after shut down. This doesn’t occur systematically, but I would say about 50% of the time.
Interestingly, when this happens, I can briefly see what seems to be a simcard icon on the top left corner of the screen (right after I select “shut down”, before the screen goes black).
Also, this problem re-appeared after I had to kickstart my battery (which, for some reason, turned completely empty after charging it for one night). I have no idea whether these two problems are linked or whether this is just a coincidence.
Have other users experienced the same issue? Is there an easy way to fix this for good? Of course I can systematically re-use the “flashlight” trick to shut it down, but this becomes very frustrating…
Thanks a lot in advance for your help!