Xamarin Forms. Http request not working on iOS when compiled for ditribution

Hi,
We are working on our first application. It uses System.Net.Http.HttpClient to connect to our backend API.
It works fine for Android, in development, emulators, devices, and with the release uploaded to the Play Store.

For iOS it works also fine on development with emulators and real devices.
But after we uploaded a distribution version to the Apple Store is being rejected by the iTunes Connect testers, because it's raising an exception when trying to access our API.

At this stage we use HTTP (not HTTPS).
For iOS we set this key in info.ptlist, in case it's related to security validations, but the issue still persists.

<key>NSAppTransportSecurity</key>
    <dict>
        <key>NSAllowsArbitraryLoads</key>
        <true/>
    </dict>

Right now we are adding some remote logging to have the exact stackTrace.

Do you have any ideas, if it could be related to the HTTP api instead of HTTPS or something else?

Thanks!