Understanding "Fixed" MAC address

Hi, I got an M3 Macbook Air earlier this year. I noticed in the settings for the wi-fi adapter there are 3 options available for the Private WiFi feature: Off, Fixed, and Random. It's obvious, to me at least, that the "Random" option will have the Macbook randomize the MAC address presented to the network. I had thought that "Fixed" would mean that the Macbook creates a random MAC address when it first connects to a network, but then always uses that address every time it reconnects. Is this not how the "Fixed" option works?

The reason I ask is that I have been using the Fixed option for my home WiFi, and I have created a DHCP reservation for the Macbook so it always gets the same IP from the router. I noticed recently that right after updating macOS, this "fixed" MAC address changed. The old one was "1e:4a:e3:--:--:--", the new one is "8e:36:3a:--:--:--". I had to go into my router and update the MAC to get the reservation back working.