I highly doubt this is "as designed". That's simply not fun and SWTOR is a game and should be fun. There's nothing more upsetting than to spend 2 hours of your night clearing a FP only to get to the climactic event of killing and looting the final boss ... only to die, not get credit for the kill, and not being able to enter the phase and loot.
If your group persists and kills the boss, you should be able to enter the instance and loot. You should get credit for the kill/completion even if you released.
I have bugged this situation twice now. Once for The False Emperor and once for Taral V.
If you do the same thing for a boss mid-FP you can enter and loot... why would it be any different for the final boss?