If the errors were "from some, but not all, apps", my first guess would be that these particular apps either don't understand IPv6 addresses, or ask the DNS onl

If the errors were "from some, but not all, apps", my first guess would be that these particular apps either don't understand IPv6 addresses, or ask the DNS only for IPv4 addresses. Both causes would stop NAT64/DNS64 or similar from working. The other apps which kept working probably can use IPv6 just fine, even if their servers are IPv4-only. The "workaround for him" was probably adding him back to the CGNAT, so that the outdated apps could see and use an IPv4 address. That kind of workaround won't be there forever; if they're starting to move phones to IPv6-only (with NAT64/DNS64 or similar), they are probably planning to turn off the CGNAT sooner or later.