2017 MacBook Pro 13in 2 Thunderbolt port Display Flickering when not plugged into external monitor

So my 2017 MacBook Pro 13 inch is having a weird issue. When it is plugged into an external monitor (with the lid open), everything is fine. When I unplug it from the external monitor, the display flickers like crazy. I've reformat the entire HDD and reinstalled macOS Big Sur and started from scratch and the issue still persist. I've started in safe mode and the flickering stops, but resumes when I go back to normal mode. I've also reset the NVRAM and SMC with no success.

Any thoughts on what else I can try?

Thanks,

Jon