Ok sambruzs, the game code is correct - there is no LOS between [17,26] and [18,25] hexes because there is a single pixel in the building [18,26] that actually blocks the path.
My game LOS engine checks LOS down to a pixel level detail, sometimes the single pixel blocking the LOS is hard to see visually with naked eye but I just double checked my code and there is no bug in the game.