I was playing with the Ribbon settings on Nauvis with hex size 24, and 4 width. It ran north-south and about 7 or 8 hexes down, there was a gap of about 4 tiles where there was no terrain - it was impassable. However, this isn't a problem since as soon as I bought the hexes, it filled it an expected.

Seed: 2216625415

Wasn't able to reproduce this. Might've been a chunk tile-filling issue. But internally the mod generates a traversable island shape for the entire planet (on each planet), and that's what is used to tell whether a hex core should be placed in nearby hexes as you claim hexes.