I'm hoping someone can help me think my way around this issue.

I have a Raspberry Pi4 set up as a media centre running OSMC. My wifi router (Archer VR400 V3) is set up to assign it a fixed IP address by identifying its MAC address. What is presently happening is that after a reboot of the Pi, it appears at an unexpected IP address, which I identify by running Angry IP scanner on my Linux PC. Also, the MAC address shown by IP scanner is not that for the Pi and the same MAC address is shown for 1 or 2 other devices on the network. At this point the Pi doesn't appear in the list of wifi clients on the router's admin pages.

I can SSH to the Pi at the IP address shown and run ifconfig to change its IP address to the value I expected to be assigned by the router. Now, the Pi appears in the router's list of wifi clients and with the correct MAC address. However in IP scanner, the previous anomalous MAC address is still shown.

I started off presuming that the reason the Pi was not assigned the expected fixed IP address is due to the anomalous MAC address, but how can it be that multiple devices appear to have the same MAC address?