WordPress.com is running WordPress.org :) So yeah, that's what I mean about how the change was in TinyMCE 4.
It is basic for any editor. Why font families without sizes? That is why a thought that it must be bug
Because in-line changes aren't really the way you're supposed to change fonds. If the middle of your sentence changes fonts, I kinda hate you ;) If you want headers and all to be different fonts, that's something you should set for the WHOLE site,not post to post. Also doing it that way makes sure when you upgrade or change a theme, that doesn't make your old choices stand out and look ... weird.