Souvenir Snatcher
Creature — Bird
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flying
Whenever this creature mutates, gain control of target noncreature artifact.
Kev Walker
Commander 2020 (C20)
• #38Rulings
Once Souvenir Snatcher has snatched a noncreature artifact, you’ll continue to control that artifact if it becomes an artifact creature.
(wotc • 2020-04-17)
The control-change effect of Souvenir Snatcher lasts indefinitely. It doesn’t wear off during the cleanup step, and it doesn’t expire if Souvenir Snatcher leaves the battlefield. In a multiplayer game, it does expire if you leave the game. If the artifact’s owner leaves the game, that artifact leaves with them.
(wotc • 2020-04-17)
Rulings
Once Souvenir Snatcher has snatched a noncreature artifact, you’ll continue to control that artifact if it becomes an artifact creature.
(wotc • 2020-04-17)
The control-change effect of Souvenir Snatcher lasts indefinitely. It doesn’t wear off during the cleanup step, and it doesn’t expire if Souvenir Snatcher leaves the battlefield. In a multiplayer game, it does expire if you leave the game. If the artifact’s owner leaves the game, that artifact leaves with them.
(wotc • 2020-04-17)