What likely happens is that someone applies numbering using the buttons on the Home tab rather than by selecting appropriate styles.

Good point!

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

I've been hunting this issue for years. It seems to be an interaction between Word, SharePoint and multiple users editing the same document. I find that when you have a document that has fairly extensive edits from multiple people at the same time, the numbering and bullets start disappearing and it doesn't stop happening from there on. Refreshing the styles from the template works, but given that first it's a procedure most users need to be told about and second that the particular template lives on my computer rather than on SharePoint (I tried that, but it's hard to link to from there) I end up being the only one who can fix it. Meanwhile, everyone else has to keep editing a document with missing numbering and bullets. The styles are all properly applied, it's just that the numbering and indents are missing. This seems to be because the list styles lose their links to the paragraph styles. I went back to a previous document and found a version that had the numbering and bullets correct (list styles on the left) and one where they were missing (the list styles on the right).

You can see the circled list styles where they are no longer linked properly. All people are doing is editing the content, not pasting anything new.