While testing different money making methods on a server that I play on, I discovered that the move thief and covet do not properly steal the held item of certain Pokemon. These items include pearls, big pearls, tiny mushrooms, and other items similar to it. This is likely because these are items that cannot be given to Pokemon once they are removed. Since the item cannot be given as a held item once removed, I figure this is what prevents the item from being stolen successfully. I tested this both in the server (where some plugins are present) and in a private, offline single-player world (where there is no plugins present) and these moves were acting this way in both scenarios. I also verified the Pokemon were holding those items by using the ability frisk to see that the wild Pokemon was holding the item, but the moves still failed to steal the opponent's item. It seems like either giving these items the ability to be re-given to Pokemon to hold or having items that are stolen in wild battles go straight into a player's inventory like pick-up, might be the best/easiest solution (though maybe still not all that easy). Hoping this bug can be fixed soon for other players that might be doing similar things in other servers, thanks!

Hey there, I was able to re-create the bug you have reported and will have it sent to our internal tracker. Thank you for reporting!

Moved to internal.