I've experienced the same issue with and lost a game thanks to it. Not sure if its the Mind Flayer itslef or its ability, Dominate Monster.
Minimus Containment description-
Enchant nonland permanent
"Enchanted permanent is a Treasure artifact with “{T}, Sacrifice this artifact: Add one mana of any color,” and it loses all other abilities. (If it was a creature, it’s no longer a creature.)
The Mind Flayer no longer has Dominate Monster and is no longer a creature. "
This should invalidate the Dominate Monster effect and therefor whatever was hit is returned to its original owner.
I've experienced the same issue with and lost a game thanks to it. Not sure if its the Mind Flayer itslef or its ability, Dominate Monster.
Minimus Containment description-
Enchant nonland permanent
"Enchanted permanent is a Treasure artifact with “{T}, Sacrifice this artifact: Add one mana of any color,” and it loses all other abilities. (If it was a creature, it’s no longer a creature.)
The Mind Flayer no longer has Dominate Monster and is no longer a creature. "
This should invalidate the Dominate Monster effect and therefor whatever was hit is returned to its original owner.