On the maps style, whilst level 0 (country) boundaries cut-off once they reach the coastline, lower level administrative boundaries wrap around over the coastline:

Creating a black outline.
This is also occuring on islands:

Instead, they should cut-off and not wrap around the coastline.

Possible solutions:

• Use a set of administrative boundaries with the symmetrical difference of the coastline and polyline boundaries rendered. Admin boundaries don't change that much so this could be seperate from updates.
• Just using a smaller line-width so the wrapping around isn't noticeable