To answer my own question, yes, it seems like they must be doing more IPV6 testing now, as my app had a bug that causes failure on NAT64 test network. The bug occurs if you specify AI_NUMERICHOST|AI_NUMERICSERV in the hints.ai_flags field for getaddrinfo(). In that case, it returns on IVP4 address even on IPV6 only network. Omitting those flags will result in a correctly synthesized IPV6 address from an IPV4 literal. The other possibility is this bug was introduced in a recent version of iOS, so may app might have passed a previous app review.
Also, FWIW I initially could not get the NAT64 test network provided by macOS Internet Sharing to work. My phones would connect to the ipv6 wifi, but no Internet access. I discovered I had "Configure IPV6" turned off under Network>Ethernet>Advanced settings. Setting this to Automatic allowed the ipv6 network to work.