Do you have an ad blocker on your phone by any chance? The app is technically just using a Chrome webview so it’s likely some sort of hook is there mangling the URL by accident? I did review all the lines of code from the string “/js” to the actual command being sent and see no way for it to gain an extra slash. Anything is possible, but it isn’t apparent to me at this moment.
The weird thing is you said the browser on the same phone works fine and the issue is localized to the app. Have you tried just deleting and re-installing the app? You can even install the latest build of the app from here: https://build.phonegap.com/apps/1373249/install if you want to test. You will need to enable 3rd party app source in order to install from there.
Thanks!