When using pistons to push either magma in place of soul sand or soul sand in place of magma, an existing bubble column does not change its direction. This can result in upwards bubble columns with magma below or downwards bubble columns with soul sand below that do not update to recognize they should be in a different direction. This even occurs when the soul sand or magma receives a block update.

Here's a video to demonstrate this bug: https://gfycat.com/PoshPolishedAntelopegroundsquirrel