Trying to place Lava between a dispenser and half slab causes the lava to be removed from the bucket, but the lava disappears

If you attempt to place a lava bucket between a half slab and a solid block the game will not let you. But if you try to place a lava bucket between a dispenser and a half slab it acts as if the lava has been placed (it disappears from the bucket) but the lava is not placed in the world