I was thinking it might have something to do with it.
But as I recall the mountpoint is defined in fstab also using btrfs - right?
I did think that advanced usage like splitting of /var /usr to separted partitions and mounting /usr readonly would create obstacles.
I had to increase my knowledge with some hands-on.
I installed my trusty test-gig using btrfs default layout, restarted onto install media and ran manjaro-chroot - it doesn’t work - just as @Aragorn mentioned.
Thank you for the enlightenment 