While playing with magma blocks under 1-wide water tunel (1 wide, several long and several height of water) I couldn't be sure where bubbles will appear. when placing magma on edges 2 furthest magmas stopped making bubbles. Imho requirements to make bubbles schould not be affected by bubbles themselves. Now bubble block column can prevent other bubble block column from creating.
Ok. Easy to reproduce: Make 2longx1widex2height water pool. Place under it 2 magma blocks, then set /gamerule randomTickSpeed 20
Observe the chaos, when bubbles appear on one side instead of other, or dissapear at all for a moment etc.