There is no order in which the instances Auto Connect. They are random. Need some method to control the order in which instances Auto Connect. There is the ability to order the connections via the 'Connection Bar Button Order' icon in the Login screen, but the Auto Connect does not match or follow that order.
So when you also have Auto Open set for windows like the Editor and Schema Browser, the order of those buttons for those windows do not coincide with the connection bar buttons.
Issue fixed in version 12.8. The latest version of Toad for Oracle can be downloaded here.