Win10 1903 & calibre 4.2
If I have these settings - I think these are the relevant ones:
If I press F11 to get into full screen, and then Winkey+Tab, I get this:
If I press F11 to get into full screen, and then PrntScrn (which fires up Snip and Sketch), I get this
If I press
any global key used by Windows, the text goes from full screen width, to the width of the viewer when its not full screen as can been seen above. the way out of the 'mess' is to hit F11, twice.
I can't get a Snip and Sketch screen shot with Alt+Tab pressed, but you should be able to work out what happens.
And I agree with the OP, Escape is the most common way to exit full screen, e.g. Youtube , Facebook, Twitter, etc, and image and video viewer apps.
Added: I never raised any of this before because I never read full screen, and if I want to read, as opposed to have a quick look at, something on desktop I use another viewer via Open With, e.g. Edge (for now) or the 3.48 viewer.
BR