I'm pretty sure it will still act consistently. Under the same compiled binary (which a hermetic environment enforces), the undefined behavior will be consistent from run to run, since the same optimizations will be used.

That's not true in a theoretical sense (undefined behavior allows me to rewrite signed overflow to "return 7 with some probability, and `rm ~/` otherwise"), but for any modern compiler, I'd assume that the optimizations chosen where signed overflow is undefined don't involve randomness.