I haven’t run into this exact issue, but it sounds like a focus management problem between bbPress and TinyMCE. It may be worth testing with the latest TinyMCE and WordPress versions or temporarily disabling other plugins to check for conflicts. I recently reviewed keyboard navigation while working on spacebartest, a browser-based spacebar testing website, and making every interactive control accessible by keyboard made a huge difference for usability. Hopefully this gets addressed in a future update.
