I think the reason for the strange HP behaviour is probably because you had engaged one before the others were popped. If you pop one but don't engage it, then pop the others, you can see its HP go down. I'd be willing to bet that had you managed to get the one you were fighting down below 50%, you'd have seen the other Diaboloses HP stay at 50% beyond that point.
Interestingly, we once popped two or three, killed one but wiped to the others and we got a title (the "Lifter of Shadows" equivalent, I forget what it was), though no ???/key item/interloper title. Then in another later run, we popped 3, again killed one but not the others and didn't get a title, so I don't know what's up with that.