Getting a stable MAC address for a RPI CM3+ with ue0 interface

Hi folks,

now that 14 is in beta and there are packages (yeah!) I try to reactivate my
TuringPi 1 cluster. This little thing is 7 RPI CM3+ with a common power
supply, one USB ethernet for each of them and a switch with a gigabit
uplink to the outside world.

I noticed that the MAC address changes with every reboot, which is
more than slightly annoying. I'd like to have the standard RPI MAC
address calculated like this for each of my CM3+:

0xb827eb000000 | (<serial number> & 0xffffff)

Linux does this by default in every distribution I came accross.
Isn't this "the way" for RPIs?

The problem: whenever I try to set the MAC address via

/etc/start_if.ue0:

/sbin/ifconfig $1 line b8:27:eb:11:22:33

I lose network connectivity. I found some older discussion (for FreeBSD 11)
suggesting to set promisc - that would match my observation that I
see DHCP requests coming from the PI (with correct MAC address) and
answers from my OPNsense, but the latter do not seem to reach the PI.

What is the correct way to get any stable MAC address and why don't
we do it like Linux does, which for once is perfectly sensible, IMHO.

Kind regards,
Patrick