I'm on a M1 Mac Mini, latest version of MacOS (11.2.1), latest version of Affinity Photo (1.9.0), no special hardware or software running, just using a normal mouse.
With the Pixel Tool selected, if I press the mouse button to start drawing on the canvas; nothing happens until I start moving the mouse. I would expect the canvas to be drawn on immediately, without waiting for mouse movement. It's been suggested that this doesn't happen on the Windows version, so it's possibly a bug, rather than behaviour that the development team intended.
I've described the bug (and included video etc) in the following thread: