DNS64 exists upstream in your ISP in the same way that CGNAT, does, in a central gateway someone along your rout path. If your CGNAT breaks, it's possible that was also your DNS64 fallback provider. For many ISPs, if you are using CGNAT still for IPv4, it probably means that they haven't even invested in DNS64+NAT64, because you can force devices to be IPv6-only and especially with most consumer devices entirely replace a CGNAT with DNS654+NAT64 today, and it is probably cheaper to do so.