The bug
If you break a magma block under a column of water the water will not flow down, if you remove it with a piston it will. I tried this in survival and creative.
Linked issues
is duplicated by 3
MC-125513
Swapping Magma blocks and soul sand keeps old bubble direction
Resolved
MC-125775
Pushing magma in place of soul sand or vice versa beneath existing bubble columns does not update the bubble column direction
Resolved
MC-126894
Water created by a soul sand/magma block's bubble column does not update after removing the block
Resolved
Confirmed for 18w07c.