I think it’s arguable whether it should do (GL’s Health+Armour)/2 or nothing, or maybe half of normal Attack damage from skulls, but definitely there’s a bug that I hope devs will address. Moreover:
Usually it is, but I’m worried about Great Maw’s third trait The animation while using Devour (from spell) looks like it deals a lot of damage to the opponent. If it works the same way with skull-Devour it could backfire somehow.
It’s slightly off-topic here, but that’s not the only ability that triggers that way. You can easily see it with Stone Skin and such (which is immaterial), but I also encountered it with Sheggra’s Molten and I think with BD’s Frozen Soul:
Definitely needs fixing too.