Most certainly you can only analyze things on boots where the problem appears, unless logs are persisted across boots. Indeed systemd-analyze focuses on boot t

Most certainly you can only analyze things on boots where the problem appears, unless logs are persisted across boots. Indeed systemd-analyze focuses on boot times, though I find it is useful to get a list of services that are run. If you are adventurous you could probably replace the umount (resp. the systemd-umount) binary by a script that runs umount (resp. systemd-umount), and also prints the process tree (and its PID) in some file so you can get insight on what ran it.