Whenever I have Word docs in SharePoint with multiple people working on them, I find that the list styles keep losing their links to paragraph styles. I have a list style called "Numbered Headings" that keeps track of how Heading 1, Heading 2 etc are all numbered. Works fine for a while, but all of a sudden someone will make an edit and none of the headings are numbered any more, and I worked out it's because the "Numbered Headings" list style isn't linked to any styles any more. If I go into the list style and link "Heading 1" back to level 1, "Heading 2" back to level 2 etc all the numbering comes back. But shortly after it will lose all the links again.
Everyone is working on the docs via the app, not in the online version of Word. They assure me all they are doing is making simple edits. This has happened to me for years with various teams so it's not just one project. A friend at another company entirely has exactly the same issue. I am trying to work out what causes it and if it's as common as it appears to be, get Microsoft to acknowledge it and fix it, because it's such a productivity killer.