Strange as that should work, I run about 6 identical boxes from the same factory i buy from and new out of the box they all have the same MAC so i use that script on each of the boxes by installing the script and then just changing the last byte of the Mac Address in the each of the boxes script and thats worked for years with no probs. To me what your describing makes me inclined to think that something further down the startup chain is setting the address... could be a plug-in of some type or some type of anonymizer or who knows but something definately adjusting it. Normally the network stack will sence the hardware's mac and use it until you some how tell it to be something else. I quess you could start by finding out what the hardware's real mac is and then once LibreELEC is up and running go look in the Info/Network panel on the LibreELEC settings addon and see what LibreELEC thinks it is. if there the same then it would appear LibreELEC is picking the network stack up ok and its something else you've got thats running and changing it.
Hard to really say without more knowing more...
As well depending on where your noticing the constantly changing address my help figure it out as well... meaning if your seeing the change happen on the box then its probably on the box but if your seeing the change further out on your network then maybe you've got something setup in your router thats spoofing the real Mac to your routers Mac for routing control or something along them lines.