After a bit of testing I can easily say that sometimes items don't disappear after getting knocked off.
I'll go straight into it and explain the testing situation:
One pokemon (Weavile) against two other Pokemons (Gyarados and Volcarona) - 1vs2
Both Gyarados and Volcarona are holding the item Leftovers.
Turn 1:
Weavile uses Knock Off against Volcarona, Volcarona loses its Leftovers.
Volcarona goes for Quiver Dance. (Just a random move, I need to stay in to get my item Knocked Off)
Turn 2:
Volcarona switches us and Gyarados comes in.
Weavile goes for another Knock Off knocking off Gyarados leftovers.
Turn 3:
Gyarados gets switched out and Volcarona comes back in.
Weavile goes again for a Knock Off, NOTHING gets Knocked Off. <--- Nothing is wrong there.
Turn 4:
Volcarona gets switched out and Gyarados comes back in.
Weavile goes for Knock Off, Gyarados loses its Leftovers even tho its item was already being Knocked Off in turn 2.
I hope the situation is clear.
To help out testing you simply need to keep knocking off while the other player keeps switching out its pokemon.
I used two Leftovers so I could have seen if the item was still there.
Just ask me if more informations are needed.
~ Xasah