Also you need to turn 'auto switch to best resolution' off.
yeah I had to do this, was getting all 720p videos hooked up to a 4k tv, they need to fix that
Yea mine defaulted to 720p too, changed it to 4K/60 no issues since.
Same to me, have to use Chromecast built-in
turned off auto select and hard set to 1080 and it's staying on 1080 for me