Unless they implemented it wrong (doubtful), deep color isn't what's finicky. It's the TV's that fuck HDMI handshake reporting.Yup, that's what I had to do as well. Deep Color is really finicky
All 'automatic' modes for HDMI query the display for EDID to determine capabilities.
The problem is display manufacturers routinely fuck up the info. That's why, thankfully, most component manufacturers offer EDID override modes (ie, directly turn stuff on/off, mode selection, etc) along with automatic.
It was pretty bad for a while though. In the early days of HDMI, many component manufactures made the assumption EDID reporting was accurate and would not offer user facing overrides. Other than maybe HDCP fuck ups, that was the single biggest problem causing devices to not work with each other. The issue becomes exasperated when you have more devices in the chain ... since they are handshaking all along the way.