...but, the
pull request doesn't seem to make any changes that should alter threading behavior, if it does it's incredibly subtle (which I appreciate is often the case with thread-safety bugs)
It does, it makes the random number generator thread specific. See the code after "// Maintain thread static state space.". Of course it could have made the RNG thread local with a much smaller commit which changed nothing except the fact that each thread got its own copy of the RNG.