Cancelling placement Events

I would think that is intended behaviour as from the game's side the event happened and then a plugin decided to cancel its action.

To avoid the player losing the blocks you would need to add them back to the inventory after cancelling the event

Can't say I remember tbh if it is different for objects

Well, actually the block isn't really removed if the event is cancelled - it just looks like the item is missing In the Java version, the client and server inventory were updated independently: When placing an item, the item is first removed from the clientside inventory, then the "place item" packet is sent to the server which invokes the API event, then - if the event wasn't cancelled - the item also gets removed from the serverside inventory and the item is placed in the world.

Cancelling the event prevents the item from being removed from the server inventory, but at this stage, it is already removed from the client inventory (strictly speaking, this results in a desync between client and server inventory). If the client closes and opens his inventory again, the game detects the desync and requests the inventory from the server - this restores the missing item (so at the end of the day, the player actually didn't really lose the item).

The reason for this overly complicated implementation was to keep the latency and bandwidth as low as possible. Tbh, this wasn't the best idea, since this implementation is error prone and doesn't work well with the API (resulting in the issue you've mentioned).

However, inventory and item handling works completely different in the new version - the client inventory only gets updated after the server inventory was actually updated (no longer resulting in desync if you cancel such an even through the API) - so yes, this will be "fixed" in the new version