Macbook Pro / Strange Failure / Primary Display Not Working If Not Connected to External Monitor

I have a display problem on my MacBook.

I use a second screen, as an external monitor, but the problem only appears on the main screen of the Mac. The main screen: either displays strange colors, or remains black, when the second screen is disconnected.

What is strange is that if the second screen is connected, both screens work fine. But when I disconnect it, the main screen no longer works. So I can no longer use the Mac without the second screen.

After trying several things, such as resetting the NVRAM or the SMC, I finally did a clean install which did not change anything either.

My MacBook is a MacBook Pro 13 retina early 2015, 2.7GHz Intel with 8 GB of Ram, an SSD drive, graphics card: Intel Iris Graphics 6100 1536 Wo. Since the clean install it is under Monterey, just before it was under Big Sur.

I add that the problem does not appear in safe mode. The main screen works well in this mode.

Do you have any idea of ​​new leads?

You can see on the pictures the problem on the main screen, when the second screen is disconnected. And on the third picture you can see that both screens work fine when the external monitor is connected.