Folks for years have found the fonts and formatting change as they open old documents. The reason why is well discussed but in case you missed it I'll give the short version.

When we open an old version the fonts may be unavailable so font changes happen. Along with this, you may not have the same printer so Word re-formats the document to give you the WYSIWYG of how it will show up given your current PC and printer.

That had nothing to do with Microsoft and folk come away a little bruised about that.

If you need "formatting" to stick around you would never use Word but a type setting program.
Bob