Bug report: subscript MML construct appears as superscript

I'd like to report a bug in the MML support which has given me trouble
before. Finally I have prepared a reproducible example.

It is attached. To see the bug:
* place the caret at the end of the document, i.e. after the "D"
* Type an open bracket using the keyboard, i.e. "(".
* Select the subscript construct from the MML menu.

What will happen is that a superscript construct will appear instead.
And the lower part of it actually appears on top of the bracket!

Thanks,
Joe