Originally those were not meant to be changed at all so hardware doesn’t always support changing it at all.

I’m assuming you’re talking about the Wi-Fi modem. Can it be changed on macOS? It should do it for Apple’s “Private Address” feature that’s on by default which randomizes the MAC, but I have no idea whether that works on that specific hardware. If it does, it’s probably some chip that Linux doesn’t know how to handle this for, is my guess. If it doesn’t, then the chip probably can’t do it at all.

ETA: on this computer there is a partition with macOS installed. Could this be the reason?

It just being installed? No

this is just general info: the other partition can’t be the reason, but fixed mac addresses in hardware were the standard decades ago. being able to change them is an optional feature that basucally has become the default nowadays. some hardware is also not able to run in promiscuous mode.

if you try to change the mac address on a device that doesn’t support it, it will usually print an error saying so. if you don’t see one, also check the kernel message buffer.

I would use OpenWRT documentation to get an idea of low level mechanisms and paths. This looks like a rabbit hole but your answer is probably somewhere around here and is just setting an alias somewhere:

https://openwrt.org/docs/guide-developer/mac.address https://github.com/openwrt/openwrt/pull/2159

Have you tried macchanger? Also don’t disable network manager,.normally you should be able to change your Mac in network manager but it sounds like maybe you’re following a tutorial of some kind that might not be right.