Using the shift key to create straight brush strokes is broken on Windows. Here's a video demonstration showing how it behaves in Photoshop when compared to Affinity:
2022-02-01 02-00-07.mp4
I also know that this is definitely broken on Windows because I have tried the Mac version in the past. These problems do not exist on Mac at all. Shift click brush strokes with a tablet work just like they do in Photoshop. Windows brush strokes need to remember the brush strength of the previous stroke, whether they are using side buttons to emulate mouse clicking or tapping with pen pressure.