This is definitely a software/driver based issue. Some update recently for me now cause the resolution to not only change, but reverse (eg: 1080x1920, 480x620 etc..) when my display (a TV in my case) was powered off. It almost looks formatted to fit on a mobile device?
Regardless, I deleted the driver (uninstall software option checked) and let Win10 use a generic driver and the problem was solved for TeamViewer. Granted I can't scale it properly on the TV with the software, but my remote session are now in 1920x1080 regardless of display power state.