BuddyPress.org

running any cache plugins?

groups_get_total_group_count() is wp_cache and is removed on the action groups_group_deleted

as for deleting a group and the associated bbPress table – correct, currently BuddyPress does not remove it from the bb_forums table (or even removing other data – topics, etc – https://trac.buddypress.org/ticket/2613 )