Hi Judderz,
I have had this before but it has only been very rare so didn't worry about it. It's not actually a '10 man defense' but shows 2 or sometimes 3 of the units as already moved forward. If you hit one of the invisible units it will show a unit on the starting back line take damage. That's because it has shown it as already moved. I often found the unit doesn't move to where the 'shadow' was. But somewhere completely different (if it was still alive after my move). It didn't appear as i say to be permanent and i can't say i have experienced it again recently (within last 5-6 months) so please try PvP again versus the same and different opponents then feedback to this thread what you find.
Thank you,
TomTom522