If you choose formatting while in sup/sub clicking the sub/sup button will make you go another level into sub/sup instead of returning to normal:

1. Click Subscript
2. Write something
3. Click underscore
4. Write something
5. Click Subscript
6. You're on another level of sub instead of back to normal

Example result code:
<p>a_{b<strike>c_d</strike>}</p>