No display on laptop but clear display on secondary monitor on a Dell Precision 7670

This is a similar question on Super User. I have recently bought Dell Precision 7670 (2023), there is no display on main screen but proper display on external monitor. All other functions are as normal, full graphic capability on 3D softwares etc. In BIOS mode, I get the display on the main screen. I have ran the advanced comprehensive diagnostic test in BIOS which took about 6 hours and no issues were identified. The problem started 2 hrs after I connected an external screen for the first time. Before that it worked fine. Dell has so far replaced the screen and motherboard but it is still the

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