If you leash a mob and place a fence post, then attempt to place another fence post against it, the lead will connect to the post and the post you are holding will appear to be deleted from your inventory. However, if you right-click with the same hotbar slot or interact with it in your inventory, it will reappear.
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I tried to recreate this in 18w44a (in survival mode), but the fence didn't disappear.
wobst.michael
Is this still an issue in the most recent versions (currently that is 1.13.2, or the latest snapshot 19w09a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.
migrated
This is still an issue in 1.21.4, and it works with any placeable block, not just fences.
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I attached a video showing the bug in action.