If water flows downwards onto magma blocks or soul sand, bubble columns will form. For example, in the screenshot below, if the blue glass were to be replaced with water, the entire glass column would be filled with bubble columns.
Downwards flowing water above magma blocks / soul sand turns into bubble columns
Bubble Columns creating water blocks from highest whole water block
Magma Block create water sources
Bubble column water pillar bug
Flowing water above magma blocks turns into bubble columns
Bubble column now doesn't generate with flowing water