EDIT: I FOUND A SOLVE. Not sure if it'll work for others, but here's what I did.
I saved my project as a new project
I changed my project setting output scaling to be what i wanted (UHD).
I setup a smart bin for timelines and disabled all timelines except for the one I wanted to change the output scaling settings for
I then duplicated that timeline and disabled the original one
On the duplicated timeline, I changed the output scaling to be UHD. Saved it.
(Not sure if this will work for changing TL resolutions, but worth a try.)
I ran into this as well. I changed my Ouput Resolution to HD for my UHD timeline so I could monitor the image on my HD monitor via my decklink card. Now, when I want to reset it to use Timeline Settings for Output Scaling, or when I change the Output Resolution in Timeline Settings from HD to UHD, Resolve gives me the spinning wheel -- i left it on overnight thinking it'd get somewhere, but eventually was forced to restart resolve.
As of now, no way to reset the output resolution back to UHD, and I believe, that if I have it set to HD here, and try to render UHD, I'll get an upscaled HD image instead of my original timeline resolution of UHD.
Can someone please advice? Will updating to 19 beta help me here at all?
Thanks
Nikhil