Suddenly lost 4-monitor display but can still run 3-monitor display

Windows 11 Version 24H2 (OS Build 26100.6725)

I have 3 external monitors connected through Displayport running on a Dell laptop in a 4-monitor setup as follows: M1: DP port on laptop; M2: DP port on Caldigit TS3 Plus dock; M3: USB port on laptop; M4: laptop display. The setup worked perfectly from the time I bought the laptop in July 2024 until yesterday.

Yesterday, all four monitors suddenly started to show the same display, the M1 display. I discovered that if I turn off M2, the system reverts to a 3-monitor display. Likewise, if I I turn on M2 and turn off M1, the same thing happens. All the monitors are working, and all the cables seem to be fine.

Thus I can now have a 3-monitor setup with 2 external monitors, but not a 4-monitor setup with 3 external monitors, as I had until something happened yesterday.

I have no idea how to troubleshoot this.

Update. Since I never used the tiny laptop screen, I turned it off with Windows key + P. After rebooting, I can see the three external monitors at the same time with different displays, which I was unable to do with the laptop screen on. Thus I have all the functionality I had before the unexplained computer failure with one exception: The home screen is stuck on M2, and it can't be moved back to M1.

On the Windows display screen, all four monitors are shown, with the laptop as No. 1, and it's grayed out and can't be moved. Rearranging the other displays in the display screen makes no difference on home screen location. It stays on M2 regardless of monitor position in the Windows display. I switched cables between M1 and M2, and that made no difference. The home screen still remained on M2.

I can live with this setup, but it's frustrating to lose basic functionality for a completely unknown reason. Typically when something goes wrong permanently and can't be fixed by a reboot, it happens after a software update or hardware change or failure. In this case, I was simply doing work at the computer when the displays suddenly all switched to the home screen display. I suppose the cause must have been some sudden failure in software or in the built-in Intel graphics in the Dell laptop.

Update. I missed the box "Make this my main display". Of course this solves the problem of the home screen on M2 instead of M1.