I think .strip() is wrong in default_slugify():

Have a look at this gist testing github slugs, you can hover the links at the left-hand side to check the correct slugs.

If there is a leading/trailing object such as an image, the stripping happens before removing the image (as in myst-parser currently). When slugifying, the image is removed, but no subsequent .strip() happens, which means slugs have a leading/trailing -.

Just removing .strip() should fix the behaviour (but maybe you want to add tests somewhere?)