Forza Apex - Won't use full screen

I’ve been a PC gamer for many years, but I’ve never come across a problem like this that I couldn’t fix; until now.

I’ve been playing Forza Apex for a while at 1280x720. Due to some recent technical difficulties I’m now running at 1360x768 (the native resolution of my screen; it’s a long story). However now whenever I run Forza Apex the image is “squashed” with black bars down the sides. I’ve set the resolution to 1360x768 and locked it in; I even tried switching back to 720p but it still shows the same behaviour. I’ve reset the app form the app manager etc, but nothing I do will fix the issue.

Halo 5 Forge works fine so it’s clearly a problem with Forza.

Anyone know hot to fix this?