Having a similar issue with v 5.5.2. I have the following setup:
Flexible Content Field
– Repeater Field
— WYSIWYG Field
When I save the page, the WYSIWYG field is blank. If I move the WYSIWYG field out of the repeater, then the content saves for some reason.
** Reverting to v184.108.40.206 has fixed the issue.