> - My left 1920x1080 monitor is
> https://www.dropbox.com/s/32f4pijigil3dgf/IMG_6260.JPG?dl=0
>
> - My right 3840x2160 monitor is
> https://www.dropbox.com/s/2advpxq1988uxac/IMG_6259.JPG?dl=0
>
> - The border between the split windows of Emacs is slightly to the
> right of the center of the right monitor.

But this means that your Emacs frame starts before the critical mark
of 3840 contradicting your earlier response

>> Does the bug happen only when the
>> entire frame is positioned to the right of 3840
>
> Yes, that's the case.

I hadn't tried split windows at that time. This time, only the right split window starts at x > 3840 and exhibits the problem, while the left one does not, as you can see in the movie.

> An additional (perhaps important) observation is:
>
> - The problem occurs only when the right display is added after the X
> server (and its first X client - mlterm in my case) started.

Did you start Emacs before adding the right display or after?

The problem occurs in both cases.