They do mention it:

> When it comes to placing block structures, there are two (2) edge cases that can throw a wrench into things:

> 1. The corners where three (3) sectors meet break the regular horizontal grid topology, since three (3) blocks meet at the corner instead of four (4)

> 2. Vertical shell boundaries break the regular vertical grid topology, since a block can have four (4) vertical neighbors instead of just one (1)

> This means that there are places on our planet where it’s impossible to define a box-shaped zone of blocks that corresponds to the block structure’s source zone. I could just detect and prevent structures from being placed at these locations, but I opted for a more general solution.

> Placing structures will now “work” everywhere, though it can get a little wonky around the problematic areas. Still, I prefer this to having dead zones where no structures can be placed at all.