Lights Out issues with new second monitor

From the looks of it, this isn't a new problem. However, it's a new one for me. I am using LR Classic on MacOS (everything the latest). Until recently, Lights Out has worked without an issue with my two monitor setup (iMac and Samsung monitor). I recently purchased a new hi-res Dell monitor and replaced the old Samsung with it. For whatever reason, Lights out is now only working on the secondary monitor. Even if I stop using the dual screen in LR.

I've rebooted. Updated. Tried unchecking the "Displays have separate Spaces" (which turned out to suck as an all-up user experience).

What's the issue? Or, more importantly, what's the fix?

LR 7.5

MacOS 10.13.6

iMac (Late 2013, 27 inch)

Dell U2718Q