Great that this topic is being discussed!
Recently my fairphone FP1U also started to have problems getting charged. This is especially annoying for me, as I already had a battery replacement because of the swollen battery and a phone replacement because of the problems with the proximity sensor like described again and again f.e. here Accidentally ending phone calls with my face.
I can only hope that all these problems are fixed in the extremly expensive FP2. Probably the “FP2” community is not as idealistic and expects a reliable phone.
Back to the battery issue. I tried several chargers, PC-USB, and several cables. But the issue seems to be caused be the charging electronics of my fairphone. It behaves as you describe, it looks like the phone is getting charged (moving charging symbol), but the percentage charged does not increase. Unplugging the phone from the cable then shows the real charging percentage (mostly less than when having it plugged in)
Neither the phone nor the charger gets warm when this behavior occurs.
Has anyone made a support request on this topic yet?
I have no solution but at least you know you are not alone.