This is still happening with 237-3ubuntu10.42 in Bionic.  RA routes announced with lifetime of 0s are not removed.  It is the firewall which used to be the default router which (continously) announce these 0s lifetimes to make sure there are no issues with failover.

$ ip -6 route show default
default proto ra metric 100 
	nexthop via fe80::21b:21ff:febc:97e6 dev eth0 weight 1 
	nexthop via fe80::21b:21ff:febc:9416 dev eth0 weight 1 

$ sudo rdisc6 eth0
Soliciting ff02::2 (ff02::2) on eth0...

Hop limit                 :           64 (      0x40)
Stateful address conf.    :           No
Stateful other conf.      :           No
Mobile home agent         :           No
Router preference         :       medium
Neighbor discovery proxy  :           No
Router lifetime           :            0 (0x00000000) seconds
Reachable time            :  unspecified (0x00000000)
Retransmit time           :  unspecified (0x00000000)
 Recursive DNS server     : 2a02:c0::1
  DNS server lifetime     :      2419200 (0x0024ea00) seconds
 from fe80::21b:21ff:febc:9416

Hop limit                 :           64 (      0x40)
Stateful address conf.    :           No
Stateful other conf.      :           No
Mobile home agent         :           No
Router preference         :       medium
Neighbor discovery proxy  :           No
Router lifetime           :            0 (0x00000000) seconds
Reachable time            :  unspecified (0x00000000)
Retransmit time           :  unspecified (0x00000000)
 Recursive DNS server     : 2a02:c0::1
  DNS server lifetime     :      2419200 (0x0024ea00) seconds
 from fe80::21b:21ff:febc:9416

Hop limit                 :           64 (      0x40)
Stateful address conf.    :           No
Stateful other conf.      :           No
Mobile home agent         :           No
Router preference         :       medium
Neighbor discovery proxy  :           No
Router lifetime           :           30 (0x0000001e) seconds
Reachable time            :  unspecified (0x00000000)
Retransmit time           :  unspecified (0x00000000)
 Prefix                   : 2a02:c0:900:113::/64
  On-link                 :          Yes
  Autonomous address conf.:          Yes
  Valid time              :        86400 (0x00015180) seconds
  Pref. time              :        14400 (0x00003840) seconds
 Recursive DNS server     : 2a02:c0::1
  DNS server lifetime     :      2419200 (0x0024ea00) seconds
 from fe80::21b:21ff:febc:97e6

Hop limit                 :           64 (      0x40)
Stateful address conf.    :           No
Stateful other conf.      :           No
Mobile home agent         :           No
Router preference         :       medium
Neighbor discovery proxy  :           No
Router lifetime           :           30 (0x0000001e) seconds
Reachable time            :  unspecified (0x00000000)
Retransmit time           :  unspecified (0x00000000)
 Prefix                   : 2a02:c0:900:113::/64
  On-link                 :          Yes
  Autonomous address conf.:          Yes
  Valid time              :        86400 (0x00015180) seconds
  Pref. time              :        14400 (0x00003840) seconds
 Recursive DNS server     : 2a02:c0::1
  DNS server lifetime     :      2419200 (0x0024ea00) seconds
 from fe80::21b:21ff:febc:97e6