ScopedForm._set_scoped_values() cleared initial['scope'] whenever the
scope_type differed from the instance value. During cloning (unsaved
instance), this runs on the initial GET and wipes the pre-filled scope,
forcing the user to re-select it.
Only clear the scope when editing an existing object (instance.pk set)
and the scope type has actually changed.
Fixes#21202
6ca82a0901