Hi!
Sorry for the late reply. We had to investigate this issue a little bit.
This is a common issue with TinyMCE, and I've seen reports on the same problem in a handful of other content management systems using TinyMCE, and none of them had a proper solution to the problem that I could see. The reasoning from the TinyMCE developers is that it keeps the output consistent between different browsers.
We'll spend some more time to try to find a fix, but I would suggest that you look for an alternative solution. I'm not convinced there is a good fix for this issue.