Make sure to clear the mbox cookie prior to testing again. I just tested on both the API and client side calls. Both seem to be functioning properly. Perhaps you should go into the Setup -> Implementation -> Debugger Tools and generate a debug token and add that to the request url. For instance:
mboxURL=https%3A%2F%2Fwww.blah.com%2F%3FmboxOverride.browserIp%3D161.185.160.93%26mboxTrace%3Dwindow%26authorization%3DTOKEN_HERE
here is the same for the api
"requestLocation": {
"pageURL": "https://www.blah.com/?mboxOverride.browserIp=161.185.160.93&authorization=TOKEN_HERE"
},
The debug will help in the sense in that it returns the override IP just after the actual IP:
"overriddenIpAddress": "161.185.160.93",
If that field is showing anything different, I would then open a client care ticket.