I took a bucket of lava and was going to place it down. The block I tapped on was a stone wall.  I expected a lava source block to form on top of the block next to the stone wall (the wall was sitting atop the ground), but the lava didn't appear on the ground. Additionally, the bucket became empty, and I was unable to place lava on the next block I tapped either. The lava source was nowhere to be found.