I know this topic is old but I experienced the same behavior...
Like you I'm also forced to use double nat as my primary WAN come from an openMPTCProuter which aggregates my WANs in front of pfSense and I also have failover to the same WANs (but in load balancing) in case of MPTCP failure so I'm also dependent on the checkIP service to update my DNS when the MPTCP gateway is the active one.
I update 4 DDNS with the same IP for this gateway and the 4 cached IPs continuously turn Red for some second, sometimes one at a time, sometimes 2 of them at the same time.
After found your post I changed the checkIP service from the default "http://checkip.dyndns.org" to the other one "https://ipinfo.io/ip" and the problem is gone.
So it's definitely a dyndns service problem, maybe that the frequency of the requests from the same IP to update 4 DNS causes the dyndns service to refuse to respond to each one.
Thanks for sharing your solution.