If I put two msubsup's right after eachother, but leave the first entry of the second msubsup empty like this:

<msubsup>
<mi>1</mi>
<mi>2</mi>
<mi>3</mi>
</msubsup>
<msubsup>
<mi></mi>
<mi>4</mi>
<mi>5</mi>
</msubsup>

The second msubsup vertically aligns as-if it connects to the second (bottom) entry of the first msubsup. If I make the first entry of the second msubsup non-empty, it vertically aligns correctly.

Thanks, looks like a bug.