Runners defined in parent groups are not listed in subgroups, probably, because there is no ‘parent group’ section. I agree this might be improved (maybe there is a feature request for it already).
But you should see both Runners in the Project.
This could happen if the other Runner was intentionally or by mistake scoped to a project (by clicking on the Enable buttons on Runner page or other means). Such Runner is scoped to Project and is not available to Group anymore (even if it was registered on Group level). Even if you remove the Runner from Project it won’t get back to Group level. At least that is my experience from the past.
If you have the option I would unregister the runner and register it again to the Group.