Have you tried eliminating the MST Hub to see if it makes a difference ?
Thank you for your reply.
Unfortunately, the monitor may have an internal MST Hub.
But it has SST mode (but it limits the refresh rate to 30 Hz), so I've tried it several times.
The primary monitor (with SST mode) once kept black out (then Windows recognized the monitors).
In addition, Windows was freezed and restarted when I switched off the monitor.