A family of Microsoft word processing software products for creating web, email, and print documents.
Are all the people who edit the document using the full version of Word for Windows (or Mac)? If someone is using the browser version, note that it has poor support for numbering and may very well break it.
There is also another thing to consider. If you are using style separators, there is a bug which may hide numbering for any heading which is followed by a style separator. This problem is known by Microsoft and it is being investigated. For more, have a look at https://support.microsoft.com/en-us/office/heading-numbering-disappears-after-applying-style-separator-in-word-535fed39-64eb-4ac1-a2c6-48f15dfd5ed5 .