A bug was discovered in Minecraft 1.21.1 whereby when placing a block in close proximity to a moving mob, the block briefly appears in the world, but then instantly disappears or breaks. This effect is observed even under standard game conditions and does not depend on the type of block.
Environment
The bug manifests itself in any game worlds of version 1.21.1 (Java Edition), both in single-player and on servers. The problem most often occurs when a player tries to place a block in the path of a mob, or if a mob passes through a newly placed block.
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We are tracking this issue in MC-163637, so this ticket is being resolved and linked as a duplicate.
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