(Rich)Text tile breaks Mosaic Layout

I have a site with some Mosaic Layouts (that has not been modified for more than a year).

When modifying the 'pages' now, the Mosaic layout breaks on save (things move into the same row).

I have a suspicion that it has something to do with (Rich) text tile, because if I remove them, it seems to save 'properly'.

Has anyone seen this before

It is possible to copy and paste such html into tinymce that it breaks the editor.

I dont think it is the case
( because some Pages has not been touche sine I måde them (the te t is very badic)