Holy post type jumping Batman! This second feature release of 2020 changes how post type archives are handled when following a post parent hierarchy. Additionally, a bug introduced in 6.5.0 was fixed where an error that would be thrown in the WordPress dashboard if the administrator role does not exist.
Previously, if the post type changed while following the post parent hierarchy, the post type of the current item was used for the post type archive. In 6.6.0, this behavior changed to use the post type of the highest level post in the post parent hierarchy. The old behavior just didn’t make sense and causes problems with plugins that use multiple custom post types in the same hierarchy. A side benefit of the change is improved bbPress and BuddyPress support.