[3.85] black line below water tiles on south elevation margins

[Galgana]

I was exploring a mountain terrain that has a mix of pine mire and open mire surrounding it.
On the mountain's south side I noticed black lines appearing under the water tiles (examining the water confirmed they are plain water rather than being springs) wherever there was a south elevation border.

These lines seem to persist at every

• zoom level
• window size
• resolution (when logical resize setting = no)
• scale quality setting