Color Management in Premiere Pro only works with GUI monitor's ICC profile
I have a 2020 iMac (P3 display) and a 1080p 24" Dell ultrasharp (sRGB) as second monitor that I use for Mercury Transmit in Premiere and After Effects.
It looks like the color management feature in Premiere Pro only look at the main display color space - the display with the menu bar. This results in having OK colors in the program monitor on the iMac screen but it darkens the picture on the Dell monitor. Meanwhile, this doesn't happen in After Effects at all, using Display Color Management in After Effects works with each ICC profile of any attached monitor. Meaning I can have OK color on both the iMac screen and Dell monitor.