I encounter this problem today using my Lr.
I discover that the Lr is displaying photos in a dimmer light and less saturated too. I only discover it when I had done with my PP and when I exported the PP'ed photos into another folder. Was shock to see that all photo is overexposed and more saturated. So went back to do a comparison on the imported photo in Lr and the one I had direct from my SD card. Was confuse to know that the imported photo in Lr looks dimmer than the one straight from my SD card.
Further compare to past project and notice that previously done PP'ed photo looks ok but the 1 showing in Lr is dimmer and less saturated too. So I come to a conclusion that my Lr is dimmer and less saturated than before. But I had no idea of what happen.
Lr is sRGB and so is my monitor.
Tried "L" mode cycle and find problem unrelated with it.
Restart my Lr and also PC, but problem persists.
Any Bro can advise whats wrong and how to rectify it.