An IPv6 router with a stateful firewall blocking incoming connections could have just the same issues with timeouts, I'd imagine. Switching to IPv6 doesn't just mean that anyone can make a P2P connection to anyone else (even STUN needs a third-party server to coordinate the two peers).
(D)TLS session resumption (I'm not sure if their "Connection IDs" are that or something similar) seems like the most foolproof solution to this scenario, assuming that the remote host can support it.
(D)TLS session resumption (I'm not sure if their "Connection IDs" are that or something similar) seems like the most foolproof solution to this scenario, assuming that the remote host can support it.