Plone 5 weird TinyMCE behavior on site

Hey All!
In case that any of you already stumbled upon something similar... We are having a problem in the site after upgrading it from Plone 5.1a1 to Plone 5.1b3.

TinyMCE started to behave in a way that is saving the content using full (complete) HTML documents (see screenshot).

This have consequences, since when the full document is rendered in Plone, then the HTML filtering happens, some things like the resolveuid links are not working, and they are being stripped from the document...

If someone has some idea, please share it! Thanks in advance!!

cc/ @esteele @tkimnguyen @tisto @ebrehault @datakurre @gforcada @pbauer @thet @jensens

FYI, already tried to upgrade to 5.1b4, no luck. Also tried to reapply some profiles, also with no success...

Did you double check the TinyMCE fullpage plugin is disabled?