UPDATE: After many hours of trying many different things, it came down to this: by first downgrading the MacPro’s software to Yosemite and then re-upgrading to El Capitan (both as clean installs), the display was again detected, and is now working fine. I think the trick is, as much of a pain as it is, when you’re running higher-end pro apps and odd displays with the MacPro, do a clean install every time you upgrade. Or risk a brick.
It’s true. I took it upon myself to install El Capitan on our 2nd edit bay this weekend, and our 4K LG Cinema display immediately stopped working with the edit machine (MacPro). The display works great with our 1st edit bay (still running Yosemite OS). And with my laptop (same).
After quite a lot of head-scratching, I’m down to this: it’s possible- not for sure- that the LG display shipped with non-VESA-compliant display port cables. So, after spending time on the VESA website, I ordered a third-party cable that IS certified.
I’ll update this as I learn more- this is a big deal for anyone who edits with these gorgeous monitors on a Mac.