Updated information!
The monitor states that it supports the following HDMI inputs:
1080p (60/59.94/50/30/25/24/.23.98)/1080i(60/59.94/50)/720p(60/50)
If I change the project time base to anything other than 23.98 then the monitor works fine. On 23.98 the monitor says "unsupported HDMI signal."
Suggests that there's something that changed with the output on HDMI at that time base. Any thoughts, Blackmagic?