Now is 2020 and I have this problem! Please Affinity finally solve it. It is really annoying to always have to close the program and reopen my file in order to continue working. The brush also stops working with a single document open and 8px brush size.
Some basic brushes still keep on working but e.g. the whole pencils-brushes stopped working.
Switching to another program and then coming back to affinity seemed to trigger it, also putting the computer to sleep, but now it also started to happen in the middle of a stroke. Restarted three times in 2 hours.
Limegreen got a reaction from GMB70 in sometimes paint brush doesn't work