Just to voice my concern about this tilemap issue as well, and the Z ordering approach in general, as a new user, the layer ordering and Z sorting has been a continuous source of confusion, having Z specified at multiple levels (game object, tilemap, layer, sprite) leads to all kinds of unexpected results.
While the object and sprite Z behavior is understandable and manageable, the tilemap layer behavior limits its usability terribly and a fix would be quite a welcomed improvement.
In the mean time, can anyone suggest a reasonable solution or workaround?