That's borders of Russia and Chukotka, screwed up.
Assigning to Paul, it's related to his borders work.
It's not clear which line this report is referring to.
The line on land is from borders, the line in the water is from the effects generated where water and land meet.
@MaxSem which line is this referring to?
Yes - which of the two lines is the issue about? They're from different sources.
We can safely say that the issue here is about what's seen of the screen. If there are two problems combined, we can always open subtasks.
Is this referring to the existing style or the new styles work? Looks like it's present in the current styles.
From the discussion I can't tell whether or not this is a bug.