I agree that it is not the overall Compatibility Mode feature; when I upsaved a copy, the issue remained.



Embedding fonts would make the file size much larger, even if including only those that are used in the document.

Round-tripping the file to .doc, .xml, and .odt and back to .docx did not shake out the issue.

I went back take another look, because I'm a glutton, and this time, instead of changing Custom to Word 2013-2021, I unticked the only custom setting in the list -- Use Word 2010 table style rules -- then OK. No change.

So I went back in to Compatibility options for, changed Word 2013-2021 back to Custom, then ticked Use Word 2010 table style rules again, then OK. That also fixed the problem.

I think that the OP's Word is somehow messing with the settings, but how or why is behind my limited knowledge of XML.

You are right, it did solve the probem. Although I still don't understand why, you helped me a lot, thanks.
It would be appreciated if anyone could give a further explanation.

When I open the file in Word 2010, one of the activated layout options is "Balance SBCS and DBSC characters" - unticking it fixes the problem.

I did a little searching and found that this option is for Asian character sets, which use DBSC, or double-byte character sets (DBCS - Wikipedia).

I think you've got Word set up to handle Asian characters, or perhaps you're actually in Asia, and your version of Word is set up to handle this type of thing out-of-the-box. In fact, looking at the XML again, I can see that Simplified Chinese is listed in settings and styles files.

When you've been creating brand new files using Word Online and you're not having the problem, maybe the online program isn't setting Asian-language attributes in the file, or maybe pasting into the only program is excluding the attributes that are causing issues on the desktop version.