Bubble columns sometimes converts horizontally flowing water to source blocks

I understand that converting the column to source blocks is intended behavior, and it is quite convenient.  However, it seems to be supposed to leave horizontally flowing water at the surface alone to preserve water streams.  This expected behavior is shown in this screenshot where I broke the blocks above where I would release the bubbles:

The problem comes in my farm where I have a roof just above the water.  The expected behavior of the bubble column stopping short of the horizontally flowing water does not happen, and my carefully built water stream system is destroyed.

This screenshot shows my water streams before I release the bubble columns:

These screenshots show soul sand and magma block bubble columns destroying the flowing water.  The one with a magma block in my hand is the magma block column:

Each screenshot involved loading a copy of my world to have an unblemished water stream to work with.