After killing Volcanic Gas for about 45 minutes, the only two that dropped the bomb queen ash were two that did self-destruct. Might just be coincidence but I thought I'd share.
Unfortunately that 2nd one killed me. :( I thought self-destruct was weaker when they have less HP, but with about 1/4 its HP left, it nailed me for 868. Funny that you get the items it drops as it kills you.