When using the internal AbstractCellForLocalCell method to check if a local cell is reachable, this should return null when the cell is unreachable. If multiple abstract cells were required for that grid, this worked as intended. Only reachable cells are stored in the localCellToAbstractCell mapping. For a grid that required only a single abstract cell, which is the common case, we optimize this to store only the single abstract cell rather than the whole mapping for potentially 100 cells in that grid. However this makes no distinction between the reachable and unreachable cells, so when we check later we get incorrect results. If a cell is unreachable but belongs to the same grid as a single group of reachable cells then we incorrectly report it as reachable. The easiest way to see this incorrect behaviour is when the PathExists is called and can sometimes indicate a path exists when it does not.
To fix this, we now ensure we perform a check to see if the cell is reachable in this single layer case, this allows us to retain the optimization where we don't need to store the whole mapping, but allows us to correctly indicate when cells are unreachable.
2a681d3791