MAC address changed after upgrading firmware every time

Hello, this problem happened on both stock firmware and openwrt(compiled by myself).
stock firmware has a fixed mac prefix, like 12:34:56:xx:xx:xx;
mac on my firmware changed randomly, xx:xx:xx:xx:xx:xx;

is there some way i can solidify mac address, thanks!

forgot the device name: Yuncore FAP680-M1

Which device are we talking about?

Deivce: Yuncore FAP680-M1