In 18w07b, Magma blocks and Soul Sand create bubble columns in which players will sink in or rise in, respectively.

Problem 1: The bubbles columns generated from magma blocks are flowing downwards.

Problem 2: Bubble columns produced from Soul Sand blocks flow upwards, yet players also float upwards.

In the real world, heated underwater sources, such as hydro-thermal vents, will produce warm packets of water or air. Because these packets are warmer than the surrounding water, they will begin rising up towards a state of lower density (ie the surface of the water). As a result, a higher density object, like a person, will sink quicker when inside one of these currents.

If a bubble stream is flowing downwards, that would indicate the material is more dense than the surrounding water above, and thus a person would find it easier to float upwards in a higher density stream.

Suggested solution: Simply reverse the directions the bubbles are moving. This will maintain physical consistency: players should expect to move upward in a downward-moving bubble stream, and downward in an upward-moving bubble stream.

tl;dr: The visual apperance of the bubble directions do not make sense given the physical behavior experienced in game.

Source: https://www.youtube.com/watch?v=uen5pMoMS5w&t=10s, https://mythresults.com/bubble-trouble, I am a physicist