"Toilet seat" drawn mask shape bugs out when self-overlapping

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

When I draw a mask shaped like, ahem, a toilet seat, the area that ends up being masked appears incorrect.

The illustration is here: https://discuss.pixls.us/uploads/short-url/9mENlEZcet729eMZ3t2W17Qlqq5.png:

1. two drawn masks - one open shape, and one with tips touching
2. an instance of exposure module with the open shape applied - all looks good
3. an instance of exposure module with the tips touching shape applied - the mask looks buggy (hashed area 4)

Steps to reproduce

Apply xmp applied to this raw to recreate the illustration linked above.

Expected behavior

No response

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.6

What OS are you using?

Windows

What is the version of your OS?

Windows 11

Describe your system

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Reproduce the mask behavior in darktable 5.6 using the linked XMP and raw image, comparing the open shape with the shape whose tips touch. Trace the exposure module's mask rendering behavior and verify that self-overlapping or touching shapes produce the intended masked area without the reported hashed region.