When changing a host's host group with the additional-classes pane open, the classes list loses it's environment. When the environment dropdown is reset, the current-applied-classes list goes to the original host group.

Pick a host.
Edit. Additional classes.
Change the hostgroup: the class list says "no environment". (The current-classes list updates correctly.)
Pick an environment: the current-classes list goes back to the earlier hostgroup

(To work around, ignore the error and save the host after changing the hostgroup, then reopen the edit page to apply any other classes.)