Mounts do not retain their reigns when loaded in after a reboot. Therefor go loose.Walking under roofs while mounted does not cause the roof to become transparent.
This has been fixed in 0.9.17
In the meanwhile, it looks like Mounts are indeed still "harnessed" when they load in, they just don't have the visual harness on their icon. In short, they're still harnessed and can be ridden.