This is probably because of physics, items are constantly colliding and they get through walls in frames where the collision is not calculated correctly. Not sure if there is a way to resolve it, also it's not really a use case in the game that players do stuff like this without having dev commands. Does it happen when you have fewer items?

Oh ive done it on the current live servers with normal loot many times. Thats how I know about it and try to remember to avoid using the back room as a loot dump.