Service umount is not enabled in medkit
umount is in
/etc/services_wanted and on update of package
base-files that contains it is enabled and started but in medkit it is not.
This looks like issue of the chicken and an egg. The
base-files packages is essentially the first package to be installed and contains also
/etc/services_wanted. It is likely that not all what is needed is available and thus service is not enabled.
This is not a big issue as the service only tries to correctly umount all mounted filesystems except of root. The same operation is automatically performed by kernel.