I'm having a peculiar problem of posting this reply. It keeps getting blocked. I'll try censoring some of the text until it goes through.
I should specify, I'm using Affinity Photo 1.0.
I have followed these instructions except to [REDACTED] instead. The app did indeed recreate those .propcol files upon restarting it, but it seems to have done nothing in resetting my brush back to default in the app itself.
Additionally, I have now tried uninstalling and reinstalling Affinity Photo and yet my brush settings persist.
Edit: Apparently the website doesn't like me posting the path to the relevant file. [REDACTED] is the location of the aforementioned file, but for Affinity Photo 1.0