As just another random instance, I updated my MacOS about a year ago and now I can only change the last 3 parts of my MAC address, the remainder appear to be fixed.
I know my hardware has the ability to change my entire MAC address - I don't get why they are doing this.
The leading octets in MAC addresses are often called "vendor prefixes", and are assigned to various hardware vendors. Apple probably wants to ensure that all their devices show up in ARP scans and MAC lookups as Apple devices.
To make it harder to spoof specific devices, perhaps. Commercial end-user OS vendors generally don't think your computer being able to do something implies you should have control over that capability.
I know my hardware has the ability to change my entire MAC address - I don't get why they are doing this.