Follow the path for packets coming in both the SSID.
Find the delta and you should find who is doing the policing on the other SSID.
UE <<>>> AP <<<>>> Switch <<<<>>>> FW <<<<ISP>>>>
Best practices.
1. Make sure you are using same device and same app to test.
2. Make sure your Device is on same AP while doing the test.
3. Check if Radius is playing a role here.