fetch() with method 'POST', not working in Android

I'm trying to use the background runner plugin, to send data in the background, and I can't send any data using a POST request, at least in Android (I didn't tested in iOS)

I tested fetch() with method GET and it works fine, but when I try with method POST, it gives me this error in Android Studio Logcat:

I leave here the repo with an app to reproduce the issue:
https://github.com/FranciskoNeves/BackgroundRunnerFetchPost

In this app you have two buttons, the first tests a fetch() call with the method GET, and the second button tests the method POST. Both are using open and free APIS.