Have you tried v19.07.0? I think the build is still in the process but it maybe available for your device as most of the targets have been built so far along with their packages.The bug may have been fixed already. If after v19.07.0 you still have this issue then you can come back here.
That creates an option macaddr to change the MAC - which works for a bridge definitely (because I tested). For a PHY (like a WiFi chip or VLAN), instead try:
config device 'xxx_dev'
option name 'wlan0'
option macaddr 'xx:xx:xx:xx:xx:xx'
xxx == the name of the Interface in OpenWrt (e.g. wlan0).
'Stock' data including MAC is stored in 'factory' partition, as far as I remember. I think, it is possible to dump it, edit, and reflash. I haven't done it.