Block Entity data is discarded when canceling BreakEvent

[Shadows-of-Fire]

When breaking a block, the client will start a prediction about the break process via MultiPlayerGameMode#startPrediction, and immediately trigger the break on the client.

While the prediction is "active", changes to the block sent by the server via ClientboundBlockUpdatePacket will not be commited into the world, instead being delayed until a ClientboundBlockChangedAckPacket is sent (which will happen next tick). This process allows the client to reset the player's position if they moved into an illegally-broken block.

Since the block change is not committed into the world, other packets that rely on the block change being committed (such as ClientboundBlockEntityDataPacket) will be discarded, meaning that when a block is broken on the client but denied by the server, the BE data is lost from the client.

This is also a vanilla bug - if you place a sign in a spawn chunk (while spawn protection is active, which requires at least one op) and break it, the sign text will go blank.