I'm experiencing the same issue in Affinity Photo (probably Designer too). In Autodesk SketchBook it works fine, so there's nothing wrong with my tablet. I'm using a Wacom Intuos Pro S.
The stabiliser makes the strokes smooth, but I can't draw like that because stabiliser causes a delay (that's how it works) so it's not an acceptable solution. Also the vector persona in Designer isn't acceptable because I want to draw with pixels, not vectors (also the strokes change a bit after they are drawn because of the smoothing algorithm).
So currently I can't draw in Affinity software.