Tiana's last ability triggers and creates a delayed triggered ability that will let you return the Aura or Equipment during the next end step. It will do so even if Tiana leaves the battlefield before the next end step.

If an Aura you control is put into a graveyard immediately after Tiana leaves the battlefield, most likely because Tiana left the battlefield but the Aura was put into the graveyard as a state-based action after it found itself not attached to anything, Tiana's last ability won't trigger.

If an Aura or Equipment you control is put into a graveyard at the same time as Tiana is, most likely because an effect destroyed all nonland permanents, you'll be able to return it to its owner's hand at the beginning of the next end step.

If an Aura or Equipment is put into your graveyard during an end step, you'll be able to return it during the next end step, not the current one.

If an Aura or Equipment leaves the graveyard after triggering Tiana's last ability, it won't be returned to its owner's hand as the delayed triggered ability resolves.