Okay, did some more work. I created a new question with the following algorthim:

$ansmathml1 = mathml("(-infinity,1/6]");

$ansmathml2 = mathml ("(-infinity, 1/6)");

$ansmathml3 = mathml ("(1/6, infinity)");

$ansmathml4 = mathml ("[1/6, infinity)");

I figured out what caused the clipping, if I don't put the variable through the equation editor and just render it in the multiple choice field as the variable, I get

So I still have my issue with the square brackets being turned into round ones. However, the clipping issue has gone away.

If I tediously edit my multiple choice so that "(-infinity," is put through the equation editor but $ansmathml is not and then back to the equation editor for a second time to get "]" I get the fraction to appear if not perfectly, than without the clipping. See

It's not perfect, but at least it is now a lot more legible.