I only use one 4k display. I use a usb3->DisplayPort cable. I’ve never had issues with using one external display with the MacBook shut. I don’t usually use it

I only use one 4k display. I use a usb3->DisplayPort cable. I’ve never had issues with using one external display with the MacBook shut. I don’t usually use it with the laptop open, maybe that is part of the issue (not that it should be).