Unable to give a potion tag for /summon potion

My friend and I are creating an adventure map and we're deciding to incorporate the new nether blocks to add effects. We finished the map and are making sure everything is stable, but we stumbled upon our "/summon minecraft:potion ~ ~-2 ~ {Potion:{id:"minecraft:splash_potion",Count:1,tag:{Potion:"minecraft:harming"}}}" command not summoning instant damage potions. This command was working prior to us updating onto the latest snapshot. We have a fair amount of command block experience and have tested multiple ways and other potions. It still continues to spawn the "Uncraftable Potion".