On 19.10 the bug does not occur and keytab entries are correct:

I joined to AD with:
realm join --user-principal=KUBUNTU-TEST$ --automatic-id-mapping=no --membership-software=samba --client-software=winbind --computer-name=kubuntu-test --os-name=Ubuntu --os-version=19.10 MPI-DORTMUND.MPG.DE

root@kubuntu-test:# klist -kte
Keytab name: FILE:/etc/krb5.keytab
KVNO Timestamp           Principal
---- ------------------- ------------------------------------------------------
   3 01.12.2020 17:04:15 <email address hidden> (etype 1) 
   3 01.12.2020 17:04:15 <email address hidden> (etype 1) 
   3 01.12.2020 17:04:15 <email address hidden> (etype 3) 
   3 01.12.2020 17:04:15 <email address hidden> (etype 3) 
   3 01.12.2020 17:04:15 <email address hidden> (aes128-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 <email address hidden> (aes128-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 <email address hidden> (aes256-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 <email address hidden> (aes256-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 <email address hidden> (arcfour-hmac) 
   3 01.12.2020 17:04:15 <email address hidden> (arcfour-hmac) 
   3 01.12.2020 17:04:15 KUBUNTU-TEST$@MPI-DORTMUND.MPG.DE (etype 1) 
   3 01.12.2020 17:04:15 KUBUNTU-TEST$@MPI-DORTMUND.MPG.DE (etype 3) 
   3 01.12.2020 17:04:15 KUBUNTU-TEST$@MPI-DORTMUND.MPG.DE (aes128-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 KUBUNTU-TEST$@MPI-DORTMUND.MPG.DE (aes256-cts-hmac-sha1-96) 
   3 01.12.2020 17:04:15 KUBUNTU-TEST$@MPI-DORTMUND.MPG.DE (arcfour-hmac) 
   3 01.12.2020 17:05:07 <email address hidden> (etype 1) 
   3 01.12.2020 17:05:07 <email address hidden> (etype 1) 
   3 01.12.2020 17:05:07 <email address hidden> (etype 3) 
   3 01.12.2020 17:05:07 <email address hidden> (etype 3) 
   3 01.12.2020 17:05:07 <email address hidden> (aes128-cts-hmac-sha1-96) 
   3 01.12.2020 17:05:07 <email address hidden> (aes128-cts-hmac-sha1-96) 
   3 01.12.2020 17:05:07 <email address hidden> (aes256-cts-hmac-sha1-96) 
   3 01.12.2020 17:05:07 <email address hidden> (aes256-cts-hmac-sha1-96) 
   3 01.12.2020 17:05:07 <email address hidden> (arcfour-hmac) 
   3 01.12.2020 17:05:07 <email address hidden> (arcfour-hmac)


So the regression starts with 20.04.


Last version of samba and realm creating a correct keytab in 19.10 are: 

root@kubuntu-test:# dpkg -l | grep -E "realm|samba"
ii  python3-samba                                 2:4.10.7+dfsg-0ubuntu2.6                    amd64        Python 3 bindings for Samba
ii  realmd                                        0.16.3-3                                    amd64        DBus service for configuring kerberos and other online identities
ii  samba                                         2:4.10.7+dfsg-0ubuntu2.6                    amd64        SMB/CIFS file, print, and login server for Unix
ii  samba-common                                  2:4.10.7+dfsg-0ubuntu2.6                    all          common files used by both the Samba server and client
ii  samba-common-bin                              2:4.10.7+dfsg-0ubuntu2.6                    amd64        Samba common files used by both the server and the client
ii  samba-dsdb-modules:amd64                      2:4.10.7+dfsg-0ubuntu2.6                    amd64        Samba Directory Services Database
ii  samba-libs:amd64                              2:4.10.7+dfsg-0ubuntu2.6                    amd64        Samba core libraries
ii  samba-vfs-modules:amd64                       2:4.10.7+dfsg-0ubuntu2.6                    amd64        Samba Virtual FileSystem plugins