I have been having a similar problem, and was hoping there would be a solution here. However, I want to add that Chrome AND Opera show the same problem (but Firefox does not). I only have this problem when posting in either a page, post, or topic on my site. Reading content that I wrote displays correctly.
I have taken additional steps including trying to reinstall all my system fonts to no avail. I have tried various fixes related to corrupt fonts on my system.
Additionally, I deleted all the files for TinyMCE and reinstalled. While I had no TinyMCE installed, I still showed the same bold italic font. I now know it not to be related to TinyMCE Advanced, but somewhere else – but I do not know where to look.
I contacted and left a support message on the theme support (https://aquoid.com/forum/viewtopic.php?f=4&t=31909), but they said about the same thing – likely a corrupted system font or a problem with TinyMCE.
At this point, I can only assume that it’s a problem with my WordPress install, but even that was recently re-uploaded. So a step in the right direction would be appreciated.
Notes:
Current version of WordPress (4.2.2)
TinyMCE (4.1.9)
Suffusion Theme (4.4.7)
Opera Browser (29.0.1795.60)
Chrome (43.0.2357.81 m)
Running on Windows 7 64bit (updated to latest updates)