Sorry, I was wrong. Resolve does switch to RGB 4:4:4 as well. It just doesn't do so until you scrub in the timeline, forcing an output refresh. So it would appear that on cards supporting 4:4:4 HDMI, it simply follows the setting of the SDI output. Is this correct?
A secondary question then. Is there any way of setting the HDMI InfoFrame metadata to specify colorimetry and range of the signal. Some monitors appear to interpret HDMI RGB data as Rec.601, but others think it's Rec.709.