I've mostly had this issue with outposts built before getting the tech that auto-generates roads, and outposts where no overland route is possible due to blocking terrain (I've cast Arcane Monolith on inaccessible islands, and in the campaign over mountains to land on the other side that I later discovered didn't have an overland route). I've also been able to cast Arcane Monolith on some Blocked tiles.
