It turns out that the set up with a multitude of additional packages is responsible for this problem. (Vanilla Emacs does not have this issue.) But it is hard to determine what exactly causes the problem.
However if the issue occurs, a manual ad hoc fix is possible: Just erase the local display table. I have defined a command for this:
(defun my-erase-local-display-table ()
(setq leo-buffer-display-table-backup buffer-display-table))
(setq buffer-display-table nil))
When things get pear-shaped in a buffer, just call this comamnd and everything should be back to normal.