Steps to reproduce (see attached video):
Find a wandering trader.
Right click one of the llamas with wheat in an attempt to feed it.
Watch the amount of wheat in your hotbar. It should decrease as if it is being consumed.
Open a chest or some other block with an inventory. Simply opening your own won't work.
The wheat should reappear in your hotbar. It is not a "ghost item" as it can be used elsewhere and crafted with.
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Can confirm. Also applies to regular llamas. In my experience, some wheat gets consumed though (I had 4 and 10 wheat not reappear).