UPDATE (on the issue of bootstrap creates 2 nics):
I reverted to version 2.6.6 of the juju snap.
"sudo snap revert --revision=8594 juju" #juju 2.6.6
Then I performed the exact same bootstrap command as I did with 2.6.8. The resulting controller now gets properly a single nic and subsequent deploys also renders into working machines.
The workaround seems to be reverting to 2.6.6.
UPDATE (on the issue of bootstrap creates 2 nics):
I reverted to version 2.6.6 of the juju snap.
"sudo snap revert --revision=8594 juju" #juju 2.6.6
Then I performed the exact same bootstrap command as I did with 2.6.8. The resulting controller now gets properly a single nic and subsequent deploys also renders into working machines.
The workaround seems to be reverting to 2.6.6.