I'm not sure if this is the solution but have you seen this question? https://forum.mendix.com/link/questions/89340
They had a white bar appearing below their status bar. I believe the mobile features widget was used to solve their issue.
Have you tried GapDebug and inspected the app? I wonder if the white bar is because of a fixed pixel height on the background? To my knowledge to modeler doesn't output any css classes that are OS specific.
I found what the issue was for me and why it was only affecting iOS. It was the iOS 11 status bar and a line of code needed to be added to the cordova config.xml during the build.
<plugin name="cordova-plugin-disable-ios11-statusbar" source="npm" spec="*"/>