Weird behaviour of Mosaic

After adding and editing a Rich Text tile in Mosaic 2.0rc8 in Tiny MCE this piece of code is added:

After each editing the same code appends. So the code is getting bigger and bigger.
This means 20 times editing adds 20 times this code.

What is this code for?

There are tons of RichText related issues with plone.app.mosaic...and I think I have seen something similar in the past.