I had posted this already on “how to”, but I rather think this is a bug.
This happened after switching from Nirvana to Parabola over the weekend.
I´ve always used TinyMCE Advanced.
I´ve deleted the troubling site below, created a new one with different URL but same content, having the same issue.
Later on created a new one with same URL as before, copying the old content, same issue.
I´m having an issue with just 1 post on our website that shows several “boxes” or independent borders within it in light grey.
It can be seen here.