A bucket of water disappears when placed on a waterlogged interactive block.

When a water bucket is used to waterlog an interactive block—such as a trapdoor, button, or similar item—the bucket disappears, and a copy of the object that was on the ground is added to your inventory.

Place a block that can be waterlogged—such as a button, trapdoor, etc.
When you click on it with a water bucket, the empty bucket disappears, and the item that was placed (the button, trapdoor, etc.) remains in your hand; this duplicates the block and causes the bucket to be lost.

The expected result is that the bucket is empty in the inventory.

But the result is that the formwork disappears, and in its place, there is a copy of the block that was installed.