Word documents lose list styles
I have a template which includes properly created list styles for headings and bullets. I create a number of documents from this template and upload them to the new Teams site for editing by the contributors. At first all is well. Eventually though, someone will tell me that the numbering is missing in a document. I go in, and sure enough, none of the headings are numbered now. If I click the list styles, the "Numbered Headings" list style is gone. To fix it, I have a macro that links back to the template, updates the styles, and then unticks the "update styles automatically" box (automating the otherwise manual process). This usually fixes it, but only for a limited time. I can keep working on the document myself for a while, and when I save it, the heading numbering disappears again (usually because there are a few other people working in the document as well).
Sometimes instead of disappearing, the numbering might turn to bullets. Or the bullets list style might suddenly turn into numbers, so the final bullet in the document is "137. Bullet content here".
Experiencing very similar issues with templates. If multiple users edit at the same time, the formatting on styles goes awry. Re-attaching the original template seems to be the quickest fix, however, it's not permanent as you say.