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.
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Stefan Blom
Microsoft Word MVP
Microsoft 365 apps for business
Windows 11 Professional
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