It seems it is really a bug as it also happens for me now.
First i loaded your track, calculated "Very Fast" shadows. Looked "Perfect" (if we can say this of that setting, but no blockies at least)
Then I calculated "High quality" shadows (not Ultra)

same bug as yours
And then after I modified one groud block and undo, the next "Very Fast" shadow calculation still had the bug
Then I cleaned the cache, loaded your original map again, "Very Fast" shadows -> nice.
Now again "High Quality" shadows, but this time with "lightindex" switched off -> still bug!
Just for a try, I used the Day mood, same result with "High Quality" shadows:
So either it is a bug, or you have too many blocks or lightsources in your map
Small tracks don't cause this issue it seems.
A investigation by Nadeo is urgently required, please
