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