Map not loading correctly in handheld version on Windows 8.1 phone

I'm working on an app that uses the handheld configuration for use on phones. It works great on Android and iOS phones, but it's not loading properly on a windows phone. In the image below, you can see that the toolbar loads correctly, but the map region is only using the upper left corner of the screen. Workflows that use display forms use the whole screen, so it's just the map region that's not working (including the results view and all of the different tabs).

/customer/servlet/servlet.FileDownload?file=00P6000000eltw2EAA

The phone is an HTC One M8 for windows running Windows 8.1. I'm using GE 4.2 and the GVH 2.3. Any thoughts on why it's not loading correctly?