Time Machine backup over IPv6

Hi community! I wonder if anyone else has experienced this issue: There is a CIFS server that can only be reached via IPv6. While accessing the server via the hostname or IPv6 address works fine from Finder, Time Machine does not seem to be able to mount the share, no matter what I do. If I add an IPv4 address to the DNS, it can mount and back up, but as soon as I remove the v4 address, it stops working. This happens even if I add the IPv4 address back and use an RFC8925-style IPv6-mostly network with CLAT, where IPv4 should be transparently tunneled over IPv6 to the NAT64 gateway.

It seems like Apple has started using its own CIFS implementation, which is lacking in terms of features.

Any ideas?