Hi everyone,
I posted something similar in the feedback forum a few months ago, but I've realised that the behaviour applies not only to grids but to all forms of snapping so I'm posting here now as a bug.
It is currently not possible to snap when constraining the curve's angle even with snapping enabled when using the pen tool normally. I've recorded a short video to show this in action.
Affinity designer snapping not working with constraint.mp4
I am using MacOS Ventura 13.1
M1 pro Macbook Pro
Affinity Designer 2.0.4
Confirmed that this bug exists on version 1 of Designer.
The only workaround seems to be to press and hold on the trackpad/mouse when drawing the curve and snapping will work, but this is really unintuitive and impractical for large amounts of drawing.