It isn't unusual at all that formatting reverts back to the document defaults. This wouldn't happen, though, if formatting had been customized properly from the start. For example, you can create one or more paragraph styles that provides formatting which should be different from the document default.



It is up to the creator of the third-party program to ensure that it continues to be compatible with newer versions of Word.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

How would I go about updating the document to properly use those paragraph styles? Note that this document is about 400 pages long, so any automated tools for fixing it up would really be appreciated, and recreating it from scratch is not practical(there's a lot of security and macros which are managed by different departments, so I can't mess with those).

Sorry, I know this one's a bit hairy.

You can change the document defaults yourself in the Manage Styles dialog box. However, whether that helps in this particular situation depends a lot on what the third-party program actually does.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional