bbPress.org

I get this issue because the BBP Threaded Replies plugin automatically de-activates the Fancy Editor.

bbPress Threaded Replies is incompatible with the Fancy Editor; enabling threading will automatically disable the Fancy Editor.

I’m not sure why they did that, but I was able to get around it by adding remove_filter('bbp_use_wp_editor', '__return_false'); to my theme’s functions.php file.