I haven't been following this, but Glenn's comment caught my eye and made me check if it was referring to me, which it was!
There's a larger issue in that this is an OPL "Library" problem (as opposed to a Contrib problem). I gave up trying to keep such problems up to date a long time ago. They were contributed at one point. Then the OPL maintainers tagged them and possibly made other edits. But at my school, we couldn't just switch to using the OPL versions because of the lag time in fixing inevitable bugs. At least at the time, back in 2013, I didn't know how I could use git better to make that possible. So the versions we use diverged from the OPL version as we fixed things. Recontributing to the OPL is a challenge because of conflicts with changes that have been made to the OPL versions (sometimes, I think, with mass edits that were done to all OPL files).
None of this is to say that the version we use doesn't have the same bug. It might. If you find the corresponding file in the OPL Contrib folder, that is closer to the one we actually use. There I can just drop our updated files with no concern about merge conflicts. It's still not necessarily what we are currently using, since maybe it has been a year or so since I last updated our corner of Contrib.
But a file diff on my system between the OPL and Contrib versions includes:
93a96
> if (defined $ansHash->{student_formula}) {
> if (defined $ansHash->{student_formula}) {
and it sounds like that is part of what was a problem here.
Anyway I thought I'd bring up this systemic issue with OPL maintenance for further discussion (but perhaps somewhere other than this thread).