Rendering issue on that chart plotter. That geometry is technically not in the data.
In the next versions of the IMC (beta), there is a polygon clean performed which cleans up these lines.
When data is clipped to a block, it's done by polygon part. On a corner that, for example, connects on the top and right of the block but not the top right, there can be a fill part that that goes to the corner and a hole part that goes to the corner. With a Inside/Outside fill, you should be able to prevent this hole section from being rendered (which NOS does but not Blue/Indigo/Violet) if you can account for this not being a border (which the current IMC does). However, if this doesn't work (because of rendering issues), your only recourse is to cut the hole out of the fill in the geometry (cross polygon part merging).
