All the issues with Japanese text input I assume are because of the Rich text Editor. You may have to turn it off in your profile so it defaults to plain text (where you can still type [ bb ] [ / code ] ).
Thing is I could try replacing it which is not easy to do, and I would have no guarantee at all that it works better, and it could work worse. If you do use a rich text editor in another website that seems to work great with the Japanese input let me know and I can investigate if it is an option for MyBB. (this one is called SCeditor).
To turn off the Rich Text Editor go to your account > Edit Options (sidebar) > check "Put the editor in source mode by default" (bottom right, last set of options). You can still toggle between "view source" and "preview" with the right most icon in the editor.
Thing is I could try replacing it which is not easy to do, and I would have no guarantee at all that it works better, and it could work worse. If you do use a rich text editor in another website that seems to work great with the Japanese input let me know and I can investigate if it is an option for MyBB. (this one is called SCeditor).
To turn off the Rich Text Editor go to your account > Edit Options (sidebar) > check "Put the editor in source mode by default" (bottom right, last set of options). You can still toggle between "view source" and "preview" with the right most icon in the editor.
Edited: Yesterday, 11:30 am