The basic problem is that when moving diagonally, the edges are ignored because edges only exist at the sides of the square, and not at the corners.
The solution to this was to query both the start tile (slg.PreTileUID) and the target tile (slg.TileUID) and from there, determine the GridMove/Board direction index that the current path was taking.
Using the principal direction (let’s assume it was a diagonal value of 6 (as demonstrated in the above image), the two other directions flanking the main one was also queried.
In the above image, the movement from the start tile is at direction 6. Direction 2 and 3 were derived from this by a lookup that defined two flanking direction for any given direction.
Then the target tile was also queried, but this time, the principal direction was the reverse of the start tile (computing for the reverse tile was made simpler by using a lookup, although a simple modulus would have done it, too). In other words, 4 is the direction. Using the same lookup, 0 and 1 were looked up as the flanking reverse directions.
Then a check is made against existing edges: are there any edges on the 2 and 3 directions of the start tile? If so, movement cost is BLOCKING. If not, then check the target tile’s 0 and 1 direction edges if they exist. And if they do block the movement.