Interesting...
My northern top walls are using this tile:
By 2,3 are you referring to the ones near the bottom left?
According to the automap mask in the github issue, it is using the "correct" tile in my screenshot; "this" cell + it's East, West, South, SW and SE are all "dungeon" tiles and there is no dungeon tile to the NW, N or NE.
The "line" one in the bottom left should only be picked if the current cell + potentially the E or W are "dungeon" and were surrounded by non-dungeon.
So it sounds like I might need to render the floor as one sheet and walls as another?
I wonder if it almost like we need another "walls" variant where instead of flat-grey under the walls, it uses a standard floor tile aligned to the grid?