Hmm... I still cannot reproduce.  Can you please post a screenshot of
how the frame looks before and after split-window-right?  I'm guessing
we use different fonts, so stuff is arranged differently on the
screen.

I have created a screencast here: https://gist.github.com/fanzhuyifan/b3221c16cda29252086bf9c0e10e8df6

Should I try forcing my emacs to use a specific font and then reproducing?