Comment # 12 on bug 1137390 from Neil Brown

> # systemctl show mnt-tahan.mount | grep Path=
> SourcePath=/proc/self/mountinfo

This means that systemd is finding out from the kernel about a filesystem that
something else mounted.

It all looks *very* strongly like something else is mounting /mnt and
/mnt/tahan and system is just the messenger.

How can you find out what?
Well... it is a bit ugly, but you could move /sbin/mount.nfs4 to
/etc/mount.nfs4.moved
and replace it with a script which collects useful data somehow, then does
 exec /sbin/mount.nfs4.moved "$@"

and see if the collected data tells you anything.
I would probably do something like
  {
   echo "$@"
   printenv
   ps axguf
  } > /tmp/logs-$$

That will create a /tmp/logs-NNN file for each NFS mount attempt, and 
report the args and a full ps listing.
>From the ps listing , you can find out process called mount.nfs.