Loupe view on second monitor is transparent

[videodave]

I'm running LR on a 2019 iMac with a Radeon Pro Vega 48 GPU, running Big Sur 11.6.8. My secondary display is a 27" LG Ultrafine. Photos shown in loupe view on the secondary display are partially transparent, allowing the content of underlying window to bleed through. This occurs both with/without GPU support enabled. Here's a partial screen shot, a browser window below the loupe view is displaying the Wikipedia home screen.

Halfway through writing this post, I switched the secondary display to full screen and the problem vanished, even when I switched back out of full screen mode. So it seems to have been a temporary glitch, but I'm still a bit concerned. Has anyone else seen this? How I might prevent it from recurring?