How to fix HDMI output issues in Final Cut Pro 11 with Blackmagic UltraStudio?

Has anyone had problems getting HDMI pics out of FCP11.1.1 on an Intel iMac Pro (OS 15.5) using a Black Magic Ultrastudio Monitor 3G as the breakout box? The box is fully working on the computer and can play clips out to monitor in Resolve and Black Magic Media Express, but in FCP, the "A/V Output" option in the View menu is greyed out, whatever I try. This always worked with my previous UltarStudio Mini Monitor until I upgraded from FCP10 to FCP11. I bought the newer box thinking this would sort out the issue.

[Re-Titled by Moderator]

Original Title: Black Magic UltraStudio Monitor 3G and FCP11