I am building a sort of trap, where a bubble column may be turned on or off at will. To this end, I have made a column of water source blocks, with soul sand underneath, and installed a dispenser with an empty bucket right next to the lowest of the water source blocks. The plan is to remove the lowest water source block via the dispenser containing an empty bucket, effectively stopping the bubbles from forming. It should be turned on again by putting the water source block back again, starting the bubble column again.

What happens is the bubble column is still there when I remove the water source block with the dispenser.

To reproduce: Create a vertical column of water source blocks, preferably encapsuled in glass blocks on all four sides. Below the lowest water source block, place soul sand. Watch as bubble column forms. Place a dispenser next to the lowest water source block. Put empty bucket in dispenser. Place button next to dispenser, fire dispenser to remove the lowest water source block. Inspect content of dispenser, observe that the empty bucket is now replaced with a water bucket.

Expected result: Bubble columns stops bubbling.

Observed result: Bubble column is still bubbling.

Further observation: Replace the water bucket in the dispenser with an empty bucket. Fire dispenser again. Observe that no water is obtained (instead, the empty bucket is dropped out of the dispenser). This shows that the water source block is no longer there, despite the bubble column still bubbling.

Further observation: Place a solid block where the lowest water source block was. Remove the solid block again. Observe that the bubble column is no longer bubbling.

(Edit: removed some lengthy duplicate text)