> GM> A few seconds grep suggests this is due to the code in newline, added ~ 3
> GM> years ago in 62ee8b1 ("Add assertions to try and help catch bug#18913").
> GM> The complete lack of progress in that report since then suggests it may be
> GM> time to remove/downgrade those assertions.
> 
> Agreed.

Make sense, so I installed the attached into emacs-26.