Swimming against a downdraft doesn't drags you consistently

When you swim upward in a downdraft, the current drag appears to intermittently cease.

Steps to Reproduce:

• Place a block of magma at the bottom of a body of water. 
• Surround the column that is formed with four columns of blocks (or just one wall on either side of the column to avoid getting out of it).
• Swim against the current.

Observed Results:

There are times when the suction of the current seems to stop affecting the player and then resume. All of this constantly happening as you go up.

Expected Results:

The suction of the bubble column should affect the player constantly and not stop intermittently.