Got 2 external monitors, through a Razer Chroma X (Radeon VII) and my 15" MacBook lid closed when working.
I use my second monitor for Program-view when editing, and first monitor for timeline, source, project-bin and such.
It's activated under Preferences -> Playback and Video Device where I have checked the desired monitor to show Program-view.

If I close, and try to open any project in any way. Premiere Pro will freeze while loading the project.
To avoid this, I have to open Premiere Pro, and before I open project I uncheck the monitor (any monitor, for that matter) in Preferences -> Playback -> Video devices. And when project opens I just re-check the monitor again.

I have a Macbook Pro 15" 2018, with monitors running on Razer X Chroma and a AMD Radeon VII, and latest 14.6.0 Build 51 installed.