I have noticed the same. Sometimes the motion detection activation itself does not trigger an alarm, in fact not too often. Perhaps my guard area looks same enough most of the time?
But I just recently started getting alerts from day-night shift. And this happens now on every cycle. Even cloud variance may trigger it, even if the camera is in well protected area, not close to windows. The strange thing in my case is that the camera has NOT been switching day-night mode, even when tested with flashlight direclty at the camera. Switching started just recentely, almost after a year of use. And now I get an alert from each mode switch.
If an update would disable (and reset) the motion detection buffer when a mode switch happens. Of course this would need some proofing, so that a flashlight would not allow entry to detection area...
I had also a 2123, and it was better with PIR detection, and more smart (?) motion detection. However that unit was defective and blind in darkness.