This happened to openqaworker4.

Installation with Leap 15.0, with /var/lib/openqa/pool on a separate partition.
The buggy read-only-root-fs did this:

grep "/var" /etc/fstab | grep -v "^#" > /etc/fstab.sys

So it copied not only the mount for /var, but also the one for
/var/lib/openqa/pool.

Later, it was upgraded to Leap 15.1, which migrated away from fstab.sys to
fstab with x-initrd.mount. It does so by moving only the mount for /var over,
which means that fstab.sys still contained the /var/lib/openqa/pool one.

Maybe transactional-update should remove other /var/* mounts as well.

IMO fstab.sys is entirely superseded by x-initrd.mount anyway and should be
declared deprecated and subsequently removed.