I don’t know if it’s a problem of TinyMCE itself or a bug in this module but I
found out the following:

TinyMCE can parse the site’s stylesheets and use these styles in the editor
field. If you have custom embedded web fonts specified with @font-face the
generated stylesheet for TinyMCE appends the URL to the module directory which
invalidates the file paths; effectively the web fonts aren’t shown and
substituted with standard fallback fonts. So, for example, if you have this line
in the stylesheet:
@font-face {
	font-family: 'Example';
	src: url(/fonts/example.woff);
}
The generated TinyMCE stylesheet makes this out of it:
@font-face {
	font-family: 'Example';
	src: url(http://example.com/modules/TinyMCE//fonts/example.woff);
}

And moreso, apparently it looks for the string “url(” to append the URL directly
after it, disregarding anything that follows. It becomes obvious if the file
path is in quotes:

src: url('/fonts/example.woff');

becomes

src: url(http://example.com/modules/TinyMCE/'/fonts/example.woff');

Now, if you are also using [[root_url]] in your stylesheet this variable is also
converted to the module directory, so:

@font-face {
	font-family: 'Example';
	src: url([[root_url]]/fonts/example.woff);
}

becomes

@font-face {
	font-family: 'Example';
src:
url(http://example.com/modules/TinyMCE/http://example.com/modules/TinyMCE/fonts/example.woff);
}