When placing a lectern under a falling block, the lectern will be in your hand and able to be placed, but the inventory will treat it as an empty inventory slot until interacting with it.

Testing this on a server, other players can see the lectern, but you cannot.

Could possibly be from the same root as the debug stick bug

Sorry about the video lag! (The world was still generating :/) Most of the video is me trying to replicate the bug, and it seems like it only happens when the lectern is placed when the gravel is already inside where the lectern should be placed

link: https://drive.google.com/file/d/1sFXeOFUFwELbPDwtzvC-H3A5lLf1NXgE/view?usp=drive_link