Trying to use this command:

/summon minecraft:potion ~ ~2 ~ {Potion:{id:minecraft:potion,Potion:"minecraft:strong_healing"}}

results in an error saying, "Unable to summon object."

This is the same error message used when trying to summon an entity with a name that does not exist, such as when misspelling an entity's name in the summon command.

However, throwing a splash potion and running this command:

/execute @e[type=potion] ~ ~ ~ /say Potion Detected!

in an activated Repeating Command Block confirms the fact that a splash potion's name is indeed internally handled as "potion," meaning that the summon command must be glitched in order to not accept "potion" as a valid name.

In Summary:

What I expected to happen:
After running the command, a Splash Potion of Healing II is summoned above my head.

What happened instead:
The game gives me an error saying: "Unable to summon object"

How to reproduce:
Run this command:

/summon minecraft:potion ~ ~2 ~ {Potion:{id:minecraft:potion,Potion:"minecraft:strong_healing"}}