If you place a block on a side of the bell that would normally ring if you right-click it, the bell will ring and the block will appear momentarily, then instantly disappear. Shift-clicking still places the block normally.
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Confirmed for 19w11b

Also, in survival the stack number decreases. It is purely visual however, as interacting with a villager, or throwing 1 item of the stack restores the number (minus 1, of course, when you throw)

Confirmed in 19w12b!
I did tests and found some extra things.
As said in the desciption of the bug, the block becomes only "invisible" of the inventory. As soon as the player makes an interaction, the block becomes "visible" again.
The player can click in his inventory to find the block but he can also click on the ground or interact with something (like a villager) to find him.
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I can confirm that the bug has been corrected!