It didn't behave like that in the past, but since a few months when a terminal window is resized, buffers are resized in a pretty weird manner.
The cursor can even locate itself outside of the displayed text.
It happens when the terminal is resized in width.
The cursor in the right hand side buffer is located inside a true line (if you yank from there and paste it inside of the left hand side buffer you'll copy something).
The display isn't trusty : a blank line can contain text.