[USG Flex H - V1.39(ABZH.0)ITS-260800686] - User Custom DDNS effective IP not appear

No but it's is even more subtle.
Returning the IP along with the nochg should theoretically not compromise other systems.
If other systems limit them to only receiving the nochg, the fact that there are other parameters attached to the "nochg" reply, shouldn't compromise them anyway, it's only a parameter and if the system ignore the parameters, you can pass 1 or 10 params, the system continue to ignore them.