Spurious bad accesses and null mPtrs on Windows

I've begun experiencing bad access and nullptr access crashes on Windows and I'm having the damnedest time debugging it. It appears to be gc-related because when debugging I'm seeing errors like this->mPtr being null, which makes little sense to me. It does not happen with -debug set either. I've been trying to narrow it down and it seems to be tied into a hot render loop that, if commented out, seema to keep the application alive, but what happens in that loop is trivial: moving an index along to replace values in a float array. Nonetheless the crashes do not happen in that loop, but they stop appearing if the loop does not run? Bizarre.

I've been trying to experiment with GC defines and disabling GC altogether but GC seems to run regardless, so I'm feeling quite lost.