Using the latest Bridge on a 2015 iMac, 32GB RAM and 10.15.4, and Bridge is usually on the second monitor.
I've had a long-standing error previewing grayscale PSDs in Bridge. No problem with RGB or CMYK. Image attached with grayscale on the left and CMYK on the right. Some weird alpha issue, I'm guessing. Problem is consistent whether I use the iMac display on its own, or have the second monitor attached. Same thing with a 2012 MacBook Pro.
Resaved files, purged preview caches and so on. Seems to be a glitch I can't solve on my end.
That does not sound good, could you please try the following steps and let us know if it helps?
Launch Bridge > open Preferences Dialog > Advanced Tab > Turn on Use Software Rendering > Restart Bridge > Now Purge cache for the grayscale files. (You can do that by right-clicking or control-clicking the file and choose Purge Cache for selection)