Make obsidian and use bucket at the same time. You can see that you have a lava bucket and an obsidian.

This is not a real lava bucket. It's actually an empty bucket. The lava poured out of the bucket will disappear and the "lava bucket" will become an empty bucket. In addition, throw away the "lava bucket" or move the items you carry, or restart the game to turn the "lava bucket" into a bucket without lava. The command block will not tested this item as a“ minecraft:lava_ bucket". But somehow, in some cases, this item will become a real lava bucket.