When the canvas rotated, the Decrease/increase brush width/hardness with on-screen readout orientation follows the rotation angle.
It is the problem with Designer's vector and pixel brushes also.
It would be great if the on-screen directions would always be the same (width: left/right and hardness: up/down).
Affinity Photo build 184.108.40.206 (220.127.116.11), Affinity Designer build 18.104.22.168 (22.214.171.124)
Steps of reproduction (Affinity Photo):
1. select any paintbrush
2. rotate canvas with 90°
3. try Decrease/increase brush width with on-screen readout with Right mouse button press+Alt+drag (left/right)
try Decrease/increase brush hardness with on-screen readout with Right mouse button press+Alt+drag (up/down)