I suppose I was responding mostly to "I'm completely thrown off on how to debug this", presuming "this" refers to the particular instance. You do have a point, perhaps it was a general question.

Unfortunately the answer is that a Heisenbug occurs because of some undefined behavior in your code whose results change depending on unrelated code (e.g. cout). In theory, any undefined behavior is allowed to result in this, but in practice many things which are undefined behavior do not have crazy effects in practice. One thing that is very often a cause of Heisenbugs is accessing uninitialized memory, because this is extremely flaky and the behavior can change even if you change a completely different part of your code.

As to how to debug it in practice, you should try to verify the bounds of all accesses. If you have some memory diagnostics/sanitizer tool or compiler options, you should try to run with those. You could also try to use debug printing as regular, but you have to treat it as if any seemingly innocent change you make results in a coin flip of whether the code crashes or not. I.e, you may have trouble printing the things you want, without changing behavior.