Covet/Thief mechanics?

In the core games, I believe that Thief/Covet works on trainers’ Pokémon (outside of battle towers etc.). However, I’ve just tried using Covet on Nora’s Celebi to get its Leftovers but, whilst I had it for the battle, it was gone by the end of the battle. Any ideas if that’s intentional or a glitch?

It’s only supposed to work on wild Pokemon.

Thanks for that. So is that a feature unique to Insurgence then? One that was altered from the main games?

No, that was changed in Gen V. You could only steal from trainers before that.

Ah I see. Forgive me, I misinterpreted Bulbapaedia. Thanks again!