Explanation is easy: with Shift, resizing also zooms the font as described in the manual.
Now if Shift is also part of a hotkey or other external function like in this case, the question is whether this can be distinguished.

Install Microsoft PowerToys - https://learn.microsoft.com/en-us/windows/powertoys/fancyzones

I don't see a download link on that page. How is it installed? And please describe the next two steps in detail for easy reproduction.

Explanation is easy: with Shift, resizing also zooms the font as described in the manual. Now if Shift is also part of a hotkey or other external function like in this case, the question is whether this can be distinguished.

Yes, this explains it, shift is also used by Microsoft Fanzy zones to "snap" to the zones.

I'm just send this in as a FYI to be nice, you do what you want with this info.

Yes, that's nice, thank you. And I'm not unwilling to handle this as a special case, if it can be distinguished. First step, however, would be that I'm getting enabled to reproduce the issue, without digging into download and installation issues myself.

There is no indication in the Windows messages (checked meanwhile) that would indicate the source of the resize request.
So it cannot be handled specifically.