While exploring with a Bombot team, I had an enemy Wight survive both explosions. With exactly 4 life each time.
It appears that the Life Drain trait triggers after Bombot does lethal damage and self-destructs, but before the code checks for whether the enemy troop is dead.
Curiously, when I tried to use a Dwarven Slayer on a Vampire Lord, things worked exactly as expected.
Is this the intended behavior? I’m inclined to say no.