To add a trivial example: if an application is coded well, it’s ready to connect to hosts both on ipv4 and ipv6, in the sense that when resolving a dns name, it will ask for addresses of any kind (unless it supports being explicitly told to only use ipv4).

So now you’re getting a record with multiple ip addresses, some of which are ipv6, but ipv6 is blocked… there you go with random connection delays and possibly timeouts.

Ipv6 exists and it’s getting more and more adoption, no matter if some people keep their head under the sand…