The fix works as expected in B/C:

#COSMIC

ubuntu@c-ufw:~$ dpkg -l | grep ufw
ii  ufw                             0.36-0ubuntu0.18.10.1               all          program for managing a Netfilter firewall

ubuntu@c-ufw:~$ sudo ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] 22/tcp                     ALLOW IN    Anywhere                  
[ 2] Anywhere                   ALLOW IN    1.2.3.4                   
[ 3] 22/tcp (v6)                ALLOW IN    Anywhere (v6)             
[ 4] Anywhere (v6)              ALLOW IN    2001:db8::/32             

ubuntu@c-ufw:~$ sudo ufw prepend deny from 6.7.8.9
Rule inserted
ubuntu@c-ufw:~$ sudo ufw prepend deny from 2a02:2210:12:a:b820:fff:fea2:25d1
Rule inserted (v6)
ubuntu@c-ufw:~$ sudo ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] Anywhere                   DENY IN     6.7.8.9                   
[ 2] 22/tcp                     ALLOW IN    Anywhere                  
[ 3] Anywhere                   ALLOW IN    1.2.3.4                   
[ 4] Anywhere (v6)              DENY IN     2a02:2210:12:a:b820:fff:fea2:25d1
[ 5] 22/tcp (v6)                ALLOW IN    Anywhere (v6)             
[ 6] Anywhere (v6)              ALLOW IN    2001:db8::/32  

#BIONIC

ubuntu@b-ufw:~$ dpkg -l | grep ufw
ii  ufw                             0.36-0ubuntu0.18.04.1                       all          program for managing a Netfilter firewall

ubuntu@b-ufw:~$ sudo ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] 22/tcp                     ALLOW IN    Anywhere                  
[ 2] Anywhere                   ALLOW IN    1.2.3.4                   
[ 3] 22/tcp (v6)                ALLOW IN    Anywhere (v6)             
[ 4] Anywhere (v6)              ALLOW IN    2001:db8::/32             

ubuntu@b-ufw:~$ sudo ufw prepend allow from 2001:db8::/32
Skipping inserting existing rule (v6)
ubuntu@b-ufw:~$ sudo ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] 22/tcp                     ALLOW IN    Anywhere                  
[ 2] Anywhere                   ALLOW IN    1.2.3.4                   
[ 3] 22/tcp (v6)                ALLOW IN    Anywhere (v6)             
[ 4] Anywhere (v6)              ALLOW IN    2001:db8::/32             

ubuntu@b-ufw:~$ sudo ufw prepend deny from 6.7.8.9
Rule inserted
ubuntu@b-ufw:~$ sudo ufw prepend deny from 2a02:2210:12:a:b820:fff:fea2:25d1
Rule inserted (v6)

ubuntu@b-ufw:~$ sudo ufw status numbered
Status: active

     To                         Action      From
     --                         ------      ----
[ 1] Anywhere                   DENY IN     6.7.8.9                   
[ 2] 22/tcp                     ALLOW IN    Anywhere                  
[ 3] Anywhere                   ALLOW IN    1.2.3.4                   
[ 4] Anywhere (v6)              DENY IN     2a02:2210:12:a:b820:fff:fea2:25d1
[ 5] 22/tcp (v6)                ALLOW IN    Anywhere (v6)             
[ 6] Anywhere (v6)              ALLOW IN    2001:db8::/32