Obviously different zoom levels give different wrong renderings of the coastline.
I would guess that the natural:coastline tag has been removed, but I don’t know how to detect that in the history… Could someone have a loot at it?
One piece of the coatsline had the wrong direction (water should be on the right side). I fixed that (sorry that I used the wrong submit comment).
Too bad that it will take a little bit until the coastline is imported again (weeks) and I hope that this should fix the issue.