Hi MondayRobot
The new text editor with the toolbar embeds the underlying <textarea> in an iframe; this is necessary to prevent clicks on the toolbar from stealing focus from the text editor. To handle styling, the system creates a dummy, hidden textarea and then copies all of the styles to the embedded textarea.
As you’ve discovered, that process doesn’t take into account custom fonts that are available in the main page, but unavailable in the iframe.
So, right now, if you want to use a custom font you’ll have to disable the new editor (there’s a checkbox in “Settings” in control panel).
Best wishes
Jeremy