Comment # 5 on bug 997197 from Steffen Winterfeldt

The problem is that leap 42.1 uses a wrong /etc/fstab. /boot/* should not be
btrfs subvolumes of /. (There's a bug report for this somewhere.)

If you check /var/log/pbl.log you'll notice that grub2-install never worked
throughout your leap 42.1 lifetime except the very first time (due to some
other bug). That's why things worked nevertheless.

A workaround would be to remove the superfluous subvolumes immediately before
the update.

Cc-ing our bootloader experts Josef and Michael.