The issue is the following:
- The stabilized normal for the main pass is calculated in
`calc_brush_plane`
- The normal is (wrongly) recalculated in `update_sculpt_normal`. This
is a common problem that other brushes have. The specific issue with the
Plane brush is that the recomputed normal is not stabilized.
- For symmetry passes, this non-stabilized normal is then assigned to
the brush in `calc_brush_plane`
This PR fixes the issue by explicitly returning false in
`sculpt_brush_needs_normal` when the Plane brush is used. A more proper
refactor would be needed, but this can easily be ported to 4.4.
Pull Request: https://projects.blender.org/blender/blender/pulls/137271
3c51029ec7