Incorrect rules interactions phasing doesn’t cause a permanent to change zones or control
During a match on MTG Arena I gained control of an opponent's creature with "Twisted Fealty" and during the second main phase after the combat phase I made the creature phasing out with "Slip Out the Back". Not being present in the game during the end phase the creature doesn't return to its owner, on my next turn it however reappears but again under the opponent's control, in my opinion it is a bug, an incorrect rules interactions, as according to the regulation, section 702.26d, which says "The phasing event doesn't actually cause a permanent to change zones or control, even though it's treated as though it's not on the battlefield and not under its controller's control while it's phased out." when a permanent is phased out it neither changes zone nor control therefore it should remain where it was before phasing out, ultimately with the combo described I should obtain definitive control of the creature.
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[Deleted User] commented
Twisted Fealty creates a gain control effect that only lasts until the end of the turn. The creature being there or not, phased in or out, doesn't matter, the effect ends in the cleanup step. With nothing to give you control over the creature, the player with the most recent gain control effect, or its default controller has control over it. So the creature phases in in your untap step and its controlled by its previous controller. Nothing wrong here.