The screenshot provided showed only one heading style linked to a level, hence my "poor advice" to remove it. If the screenshot had shown links to multiple heading styles, I would have provided other advice.

The sample file has 21 multilevel list styles in it (determined via macro).

It is very easy to continually and unwittingly create new multilevel list styles by clicking Define New Multilevel List. These are then not accessible via the Multilevel List button. They live in the file, unnamed, they accumulate, and you can't search for where they're applied.

(In addition, in the List Library, Word shows multilevel list styles from other open files, which can add confusion.)

What I would do is create a new, NAMED multilevel list style, set up the levels/headings to work as needed, then always use that named list style, and never use Define New Multilevel List when modifications to the list style are needed; instead, only modify the named style directly.

When you have a named list style, it will appear via the Multilevel List button in a section called List Styles, below the List Library thumbnails. Modify a named list style by right-clicking > Modify.