Monitors not being detected when a Mac wakes from sleep can sometimes happen when the monitor is using automatic input source detection. Depending on the manufacturer of the monitor, the name of this input source or setting may vary. Some of these include “Auto Select”, “Auto Detect”, “Auto Source”, or other various combinations including “Auto” in the name. 

The use of automatic input source detection can be problematic because it causes the monitor to be slower to inform the computer that it's connected when this setting is enabled. The additional delay introduced by automatic input source detection can cause the monitor connection negotiation between macOS and the monitor to time out, leading macOS to think there is no monitor connected to the port. As a result, video will not be sent to the monitor, and the monitor will not be detected by the Mac.

The solution to this problem is to disable automatic input source detection on your monitors. The method of accomplishing this will also vary by monitor model, however there are a few common methods such as:

• Setting the monitor's Input Source to the specific connection being used rather than automatic detection
• Disabling automatic input source detection within the monitor's on-screen display (OSD) settings

For more details on how to disable automatic input source detection, please consult the documentation for your specific monitor model from its manufacturer.