My best guess is that this is because we have enabled the html 5 schema by default in TinyMCE and their main demo site uses html 4.
You could override it in mojoPortal by adding this to your user.config file:
<add key="TinyMCE:Schema" value="html4" />
using the latest TinyMCE.
They do have an Html 5 demo, and it may behave the same way I have not tried it and not sure if they keep that one updated.
Hope that helps,