Soulsand elevator does not dissapear after removing water

My friend and I made a nether wart farm and I realized that when a dispenser dispenses water onto soul sand then removes the water, the bubbles remain.

How to replicate:
1.) Place a block of soulsand
2.) Place a dispenser that will dispense water onto the soulsand.
3.) Place a button on the dispenser
4.) Press button twice
5.) Enjoy you have bubbles without water