Same problem here (with gapps and Magisk). I updated also my LOS 16 to 17.1 and the MAC changes at every reboot. But i have another device which was newly installed from scratch which doesn't have that problem. The MAC keeps the same. May be some kind of update problem?
I have done some tests. It looks like expected. If i update from LOS 16 to 17.1 the MAC always changes. If i wipe /data before the MAC keeps the same. But with wiping data an update makes no sense.... So something on data results in that problem, but i don't know what. Any ideas?