Here's the anomalies: Here's the preview given by the feudal tools generator: The lines don't exist in the heightmap. I've generated several times based on the same general map concept and sometimes it's there, and sometimes its not, but they always pop in at the same point.

The top image is "far" north, but not all "most" north, so its not being cut off by the map limits. The second is an island somewhere in the south east, and only cuts through the island, but doesn't cut through anything else.

Guidance would be extremely appreciated.

I have seen this before. It's a rare bug with the game's world generation. It's not a problem with your heightmap.
If one region (terrain block) does not have enough landmass, the northernmost couple of tile rows won't be rendered.
There is no good solution to this other than adding more landmass to the bugged terrain block.