AFAIK this is an issue with older versions of Samba, and that they should be upgraded to >4.16.2 (2002-06-13). Apparently Synology has still not integrated these updates into their Directory Server product.
This Samba issue was initially noticed with the Dev Preview back in January, and although Samba addressed it with an update in June, it seems that the issue has not been resolved w/Synology.
Some months back it was determined that the update is enforcing Kerberos encryption during the authentication process. The fix for that is to enable "Use DES encryption for this account" on the accounts, but this can potentially break other related things in the environment.