Well, for different reasons, but you have similar issues with IPv6 as well. If your client uses temporary addresses (most likely since they're enabled by default on most OS), OpenSSH will pick one of them over the stable address and when they're rotated the connection breaks.

For some reason, OpenSSH devs refuse to fix this issue, so I have to patch it myself:

    --- a/sshconnect.c
    +++ b/sshconnect.c
    @@ -26,6 +26,7 @@
     #include <net/if.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>
    +#include <linux/ipv6.h>
     
     #include <ctype.h>
     #include <errno.h>
    @@ -370,6 +371,11 @@ ssh_create_socket(struct addrinfo *ai)
      if (options.ip_qos_interactive != INT_MAX)
        set_sock_tos(sock, options.ip_qos_interactive);
     
    + if (ai->ai_family == AF_INET6 && options.bind_address == NULL) {
    +  int val = IPV6_PREFER_SRC_PUBLIC;
    +  setsockopt(sock, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES, &val, sizeof(val));
    + }
    +
      /* Bind the socket to an alternative local IP address */
      if (options.bind_address == NULL && options.bind_interface == NULL)
        return sock;

This is a very common misconception. The issue is not IPv4 or CGNAT, it's stateful middleboxes... of which IPv6 has plenty.

The largest IPv6 deployments in the world are mobile carriers, which are full of stateful firewalls, DPI, and mid-path translation. The difference is that when connections drop it gets blamed on the wireless rather than the network infrastructure.

Also, fun fact: net.ipv4.tcp_keepalive_* applies to IPv6 too. The "ipv4" is just a naming artifact.

Mobile carriers usually have stateful firewalls for IPv6 as well (otherwise you can get a lot of random noise on the air interface, draining both your battery and data plan), so it's an issue just the same.

The constrained resource there is only firewall-side memory, though, as opposed to that plus (IP, port) tuples for CG-NAT.