After entering a line of text in two column mode the merge operation 
leaves the second column window showing

  . http://sdf.org/~van.ly/img/emacs-2C-merge--fails.jpg

Unexpected result in the above picture shows second column window.

Steps to reproduce

  . start "emacs -Q"
  . open file "/dev/shm/text.text"
  . enter two column mode by "F2 2"
  . enter a line of text in two column windows
  . merge the two column windows by "F2 1"

Expected result has one window with one line of text in two columns.

-- 
vl
 				 ALPINE 2.24 GNU Emacs 27.2 NetBSD 9.3