Hi there,
Many of my imported brushes from Affinity photo V1 weren't "working" in V2.
So I carefully matched the brush settings for the brushes in question in the brush panels between V1 & V2. The settings were the same and so nothing had changed....The solution was to change the flow jitter in the Dynamics panel down from 100% to ~ 35-50% settings and the paint strokes appeared ,,yeah!
Hope this helps if you should run into this issue.