I’m having a similar problem. But it only is happening to fonts in IE and FF that are addressed in my Child Theme or fonts that have been altered (bold, color NOT font type) in the tiny WSIWYG. In my child theme I have NO references to font type only size and color.
If I add the CSS used in the CSS generated by this app to my child theme, most of the issues disappear in FF but not IE. I say most because the main navigation is still unchanged.