“Unreproducible” can also be read as “Wasn’t worth the effort to reproduce”.
Very, very rarely will a Sev1 crash in production software be instantly reproducible. When it is, it’s an utter embarrassment. More often the full set of conditions which lead to the crash are unknown, rare, and hard to fully quantify all at once.
And yet, we know with certainty that the crash did happen and by definition it is a valid bug in any sane world where “don’t crash” is an absolute requirement.
Any crashing bug in a hypervisor must be assumed to be security critical until proven otherwise. Even after “proving” a crash can’t possibly be exploited, you should still assume that a clever attacker will figure out a way. The Venn diagram of “crashing bugs” and “security exploits” is vanishingly disjoint.