When making availability report using Host mode and filtering by specific host group, query is selecting all nested Host groups that has nothing to do with filtered one.
Create Host group - Test group ( with host "1" inside it )
Create Host group - Nested/nested1 ( with host "2" inside it )
Create Host group - 1_Nested/1_nested1 ( with host "3" inside it )
Open Reports -> Availability report page.
Query from debug: