A small addition.
Apple platforms also use a randomized MAC address when conducting enhanced Preferred Network Offload (ePNO) scans when a device isn’t associated with a Wi-Fi network or its processor is asleep.
This feature can be disabled either by the user or using a new option in the Wi-Fi payload. Under certain circumstances, the device will fall back to the actual MAC address.
On some Android smartphones, mac address randomisation can also be blocked.
So if these are your smartphones, you can turn off randomisation for your wifi network on them.
I hope I have written this understandably 
Best.