Yesterday during row D maintenance we ran into an issue similar to T133387 where hosts would lack ipv6 connectivity but ipv4 was working correctly. Icinga didn't alert about lack of ipv6 connectivity but it should have IMO, in case hosts have A and AAAA records.
i noticed i had this comment that i started typing but never saved:
currently not going to work on this and we should probably wait a bit closely watching performance metrics before we add so many extra checks
meanwhile there have also been efforts to add a lot more AAAA records to DNS and to add the mapped v6 address