Just created an account to say that @spectacula is 100% right.
The issue comes from the ‘user_can_richedit’ check.
Even if the value is set to true in the user profile or database, WordPress will ignore it based on the user agent string (and probably lots of other undocumented cases).
The solution is to add a filter function in wp-config.php like the example @spectacula posted.
Thanks!
