I did some more testing. It looks weird because max health of a big worm is just under the threshold so regeneration gets there in the time between the 1st and the 3rd rocket to trigger a 4th. I agree with you that it's the same thing other people posted. Sorry!

I am unable to shoot more than enough (yellow) rockets at once to kill a worm, but at the moment the last killing rocket reaches it, spidertron and handheld rocket launcher are able to unnecessarily shoot it one additional time. I haven't tested any other weapon or ammo.

I'm attaching my save file.