What you were expecting to happen, and what actually happened:
The giant gem explodes a doomskull, which explodes as well. I was expecting the front opponent troop to receive 5 damage from the doomskull. It didn’t receive any damage at all.
Might possibly also affect the amount of mana gained from other gems caught in the explosion, needs further analysis.
Looks like the same visual bug where damage is applied correctly but isn’t updated graphically. It should correct itself next time the troop takes damage
I’ve witnessed a few cases now, where enemies died at the end of my [Edit: Or due to effects like poison and burn, sometimes their] turn because of damage, that was caused by a giant gem explosion earlier. So as stated above, it is a visual glitch, possibly including a calculation delay, but the damage is not lost.
P.S.: As usual with small numbers and their influence, it is the easiest to replicate it in the arena.