An IPv6-only network normally has NAT64 and DNS64 for access to IPv4-only websites. DNS64 returns synthesised AAAA records for any domain which does not have one. This involves an IPv6 prefix (usually 64:ff9b::/96) and packing the bits of the IPv4 address into the last 32-bits of the IPv6 address, for example it can be written as 64:ff9b::22.214.171.124 or 64:ff9b::101:101 to access IPv4 address 126.96.36.199 on an IPv6-only network, assuming the NAT64 prefix is 64:ff9b::/96. The network routes packets with this prefix to a NAT64 gateway, which has native IPv4 access, which turns the IPv6 packets into IPv4 packets and transmits them on the IPv4 internet. It reverse-translates returning IPv4 packets into IPv6 and transmits them back to the originating device on the IPv6-only network. It creates the illusion that every domain has an AAAA record and is accessible via IPv6.
The issue is if a domain publishes a broken AAAA record (if IPv6 goes down on their server, for example). There will be no synthesised AAAA record for that domain, because a real AAAA record has been published. It means that IPv6-only networks cannot “fall back” to IPv4. The only way fall-back can work on an IPv6-only network is if the device is running a CLAT as part of 464XLAT.
The problem for community[dot]cloudflare[dot]com:
It appears that sjc4[dot]discourse-cdn[dot]com and aws1[dot]discourse-cdn[dot]com have these issues (they each have AAAA record, but are not reachable via their respective AAAA records), and are required for community.cloudflare.com to load. I have had to get on an IPv4 / dual-stack network just to make an account to post this. Can Cloudflare please contact the domain owners to sort this out? It means any device on an IPv6-only network (common for 4G mobile deployments) without 464XLAT will not be able to load community[dot]cloudflare[dot]com.
Reproduce the problem for yourself:
Anybody with IPv6 internet can easily try it, grab a DNS64 server address from nat64[dot]net and use their public NAT64 implementation. Override your IPv6 DNS Server on your internet network interface to the address you obtained, and disable IPv4 on that same interface. You should be able to access IPv4 and IPv6 websites, but community.cloudflare.com will only load to a blank white screen. You can go into developer tools of the browser and see the connections which are failing. If you own a Mac, here are directions for using it to create a NAT64 test network (intended for developers to be able to check if their apps work behind NAT64 gateway): developer[dot]apple[dot]com/forums/thread/4971
Thanks to anybody willing to take me seriously.