Issue is non-repro in the latest oxygen build, as confirmed by the submitter so resolving the bug.

cli>display fib
   RD       Prefix                NextHop               Label    Origin
   -------------------------------------------------------------------
   fb444e8b-3ae2-4d7d-942a-84c3343e9ade  2001:db8:1111::1/128  0.0.0.0               103001   l
   fb444e8b-3ae2-4d7d-942a-84c3343e9ade  2001:db8:1111:0:f816:3eff:feed:c06/128  192.168.115.113       103057   l
   fb444e8b-3ae2-4d7d-942a-84c3343e9ade  2001:db8:1111:0:f816:3eff:fe01:ef36/128  192.168.115.113       103059   l
   fb444e8b-3ae2-4d7d-942a-84c3343e9ade  2001:db8:1111:0:f816:3eff:fee9:2139/128  192.168.115.114       103055   l
   fb444e8b-3ae2-4d7d-942a-84c3343e9ade  2001:db8:1111:0:f816:3eff:fed3:155a/128  192.168.115.114       103053   l

cli>

cli>exit
Quit/Exit is called, process is terminating
root@cic-1:~# neutron router-list

root@cic-1:~#

Resolving the defect as non-reproducible, as confirmed by the submitter.

All the fib entries and flows are getting cleared when ipv6 subnet is removed from the router.

 Table 210(INGRESS_ACL_ANTI_SPOOFING ) and table 240(EGRESS_ACL_ANTI_SPOOFING) maintains entry for ipv6 ip-address which is assigned to VM triggered, this entry in the table is removed when the VM is deleted.  This is expected behaviour.

neutron router-create R1
neutron net-create mynet1
neutron subnet-create mynet1 10.1.1.0/24 --enable-dhcp --name net1-snet1
neutron port-create mynet1 --name myport11 --security-group=CUSTM_SGP
neutron port-create mynet1 --name myport21 --security-group=CUSTM_SGP
nova boot --image cirros --flavor myhuge --nic port-id=$(neutron port-list | grep 'myport11' | awk '{print $2}') myhost11 --availability-zone nova:compute-0-4.domain.tld
nova boot --image cirros --flavor myhuge --nic port-id=$(neutron port-list | grep 'myport21' | awk '{print $2}') myhost21 --availability-zone nova:compute-0-5.domain.tld

neutron subnet-create mynet1 2001:db8:1111::/64 --ip-version 6 --ipv6-ra-mode slaac --ipv6-address-mode slaac --name net1-snet2
neutron router-interface-add R1 net1-snet2
neutron port-create mynet1 --name myport12 --security-group=CUSTM_SGP
neutron port-create mynet1 --name myport22 --security-group=CUSTM_SGP
nova boot --image cirros --flavor myhuge --nic port-id=$(neutron port-list | grep 'myport12' | awk '{print $2}') myhost12 --availability-zone nova:compute-0-4.domain.tld
nova boot --image cirros --flavor myhuge --nic port-id=$(neutron port-list | grep 'myport22' | awk '{print $2}') myhost22 --availability-zone nova:compute-0-5.domain.tld

With latest oxygen and fluorine build issue is not re-producible. The same thing is communicated to bug submitter and issue is not even re-producible in bug submitter setup as well based on the confirmation from bug submitter. 

Hence closing this issue.