Just in case it is the power button’s fault (not for everybody in this thread until now, but just in case) …
Fairphone acknowledged a problem with the slim case and its power button for a number of customers.
If your power button is giving hints of not working reliably, you may contact Fairphone about it …
Or you can physically swap the power and camera button of the slim case yourself and see whether that works better (I did that, too, because of missing feedback from the power button whether I actually pressed it enough or not, and it helped) …