Unity Version: 2020.01.2f1
AStar: 4.2.15
While using List<GraphNode> reachableNodes = PathUtilities.BFS(node, distance); It seems to add an extra node or two that it should not when scanning. Here is an example of what I am seeing: 
The extra tile is marked with a red circle.
A second example moving the unit down 1 tile and showing the extra tile extended off the end of the max range. 
This is the full code I am using.
public class ShowMoveableGrid : MonoBehaviour
{
public GameObject thing; // Holds prefab
public GameObject thingHolder; // Parent for prefabs for tracking
public BlockManager blockManager;
private BlockManager.TraversalProvider traversalProvider;
public void UpdateUsingCurrentPos(int distance, Transform _transform, List<SingleNodeBlocker> obstacles)
{
GraphNode node = AstarPath.active.GetNearest(_transform.position).node;
traversalProvider = new BlockManager.TraversalProvider(blockManager, BlockManager.BlockMode.OnlySelector, obstacles);
List<GraphNode> reachableNodes = PathUtilities.BFS(node, distance);
foreach (GraphNode nodeToSpawn in reachableNodes)
{
Vector3 ourPos = (Vector3)node.position;
Vector3 newPos = (Vector3)nodeToSpawn.position;
Path path = ABPath.Construct(ourPos, newPos, null);
// Make the path use a specific traversal provider
path.traversalProvider = traversalProvider;
// Calculate the path synchronously
AstarPath.StartPath(path);
path.BlockUntilCalculated();
if (!traversalProvider.CanTraverse(path, nodeToSpawn))
{
continue;
}
if (path.GetTotalLength() < distance)
{
var v3 = (Vector3)nodeToSpawn.position;
Instantiate(thing, v3, Quaternion.identity, thingHolder.transform);
}
}
}
}