Since snaphot 529 (604 now, skipped few before, sono not sure since when..) one
o my partition is mounted and unmounted during boot.

# dmesg | grep XFS:
[    5.279229] SGI XFS with ACLs, security attributes, no debug enabled
[    5.280308] XFS (sda2): Mounting V5 Filesystem
[    5.280487] XFS (sdb5): Mounting V5 Filesystem
[    5.327015] XFS (sda2): Ending clean mount
[    5.396838] XFS (sdb5): Ending clean mount
[    6.382957] XFS (sdb5): Unmounting Filesystem

# fstab (created at installation time by yast, default subvolumes removed
here..):

UUID=565e589b-71ff-4d5a-b289-59b29f66c181  /                        btrfs 
defaults                        0  0

UUID=44dccc07-9527-4ea7-8e15-5dc63486a9cd  /local                   xfs   
defaults                        0  0
UUID=99c78e04-da7c-4291-9944-1e11510d8af8  /home                    xfs   
defaults                        0  0
UUID=71043c82-a659-4e7b-8a2e-0a5645c424f7  swap                     swap  
defaults                        0  0

Nothing usefull from journactl:
giu 05 18:34:05 fyrfos kernel: XFS (sdb5): Mounting V5 Filesystem
...
giu 05 18:34:05 fyrfos kernel: XFS (sdb5): Ending clean mount
giu 05 18:34:05 fyrfos systemd[1]: Started udev Kernel Device Manager.
giu 05 18:34:05 fyrfos systemd[1]: Mounted /local.
giu 05 18:34:05 fyrfos systemd[1]: Reached target Local File Systems.
...
giu 05 18:34:06 fyrfos systemd[1]: Stopped target Local File Systems.
giu 05 18:34:06 fyrfos systemd[1]: Unmounting /local...
...
giu 05 18:34:06 fyrfos kernel: XFS (sdb5): Unmounting Filesystem


# mount -a "fix" the problem till next reboot I can also mount the partition
with Dolphin.

Fs is clean, hdd is ok.
I've this setup since more then one year.
So.. it's weird.