The bug

When water or lava source is clearing, water or lava spread doesn't clear all the way unless they receive random ticks. It appears to be the level of snow layers.
A block can be placed to clear it. A torch can also be placed on it to remove it.

How to reproduce

• Create a world with seed -6222383818302921049
• Disable random ticks (/gamerule randomTickSpeed 0), to simulate a fix for MC-93468
• Remove the lava source block at 247 17 -27