Hi
Prioritize graphs does not do what you think it does.
Here is the (a bit light I guess) documentation: http://arongranberg.com/astar/docs/class_astar_path.php#a565b110e4b10f36bc9d8cb8573599a33
So your agent on the grid graph tries to find a path to a point on the road. However since prioritize graphs is enabled, it will find that the road graph is closer and pick a node on the road graph as the target node. But the road graph and the grid graph are not connected. There is no information available about how to move from the grid graph to the road graph so it tries to find a path, but it does not find one.
My suggestion is that you connect your road graph with the grid graph.
You can do this by in the Scan method of your road graph, register to the AstarPath.OnPostScan delegate to call a new method called say ‘ConnectGraphs’ (because you need to wait for the grid graph to be scanned). Inside ConnectGraphs you unregister from the delegate and then for each node in your road graph, try to connect it with the closest point on the grid graph (or a few of the closest nodes which will likely give you slightly better path quality). You can do that using a AstarPath.active.GetNearest query with an NNConstraint where the graphMask is set to only search the grid graph (the graphMask is a bitmask so if it is set to 1 << 2 for example, it will only search graph index 2, in your list it looks like the grid graph is graph index 1, so you should set it to 1 << 1).