I can't diagnose the cause from a video but it appears that the problem is coming from the user's Normal.dotm. I would delete that template from the machine and let Word recreate it.

If you examine the Manage Styles settings you might spot something odd on the Heading style (eg Restrict tab settings) although I would also check the style above Heading 1 as the settings of your new style looks like it was based on "No spacing" rather than Heading 1. Is it possible that Teams offsets the pixels so what you think you are clicking on is actually not what you are clicking on?

If docs created on another machine are opened on that user's machine, does the same issue appear? What happens if you save that doc to your machine - does it show the same issue on your machine?

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Andrew Lockton
Chrysalis Design, Melbourne Australia