Hello,
Making quick short brush stroke (either by mouse or wacom pen) draws a single dot instead of a short line.
It looks like when you start a brush stroke, Affinity Photo checks if it was a single "click" or a longer brush stroke by waiting until you either move the mouse/pen by a short distance or if a small timeout expires (whichever happens first). If the brush stroke is shorter than the threshold or you release the mouse/pen sooner than the timeout, Affinity Photo fill draw a single dot instead of a short line.
The distance threshold seems to be dependent on Windows DPI scale setting (Windows->Display Settings->Scale and Layout). The larger the UI scaling is, the larger the distance threshold. With 100% DPI scale the effect is less noticeable, but still there.
The "High precision tablet input" in Tools setting has no effect on this. Other painting programs don't have this issue and it doesn't look like a tablet issue because it can be reproduced with a mouse.
This makes drawing and sketching small details really annoying.
It seems to be related to the bug where clicking leaves brush marks when panning with space bar. Whether Affinity Photo leaves the marks or starts panning is dependent on the same distance threshold and timeout mentioned above.
Setup:
Windows 10 with Display Setting DPI Scale 200%
4K Monitor: 3840x2160
Affinity Photo Beta 1.6.0.84