What were you doing when you found the issue?
I gave Dark Harvest LeBlanc a try earlier and found that if her Mirror Image timed out I gained a stack of DH. I did some testing and found that Illaoi's Test of Spirit does the same if it times out or the target leaves the tether range. Shaco's clone provides a soul if it times out. Mordekaiser's Children of the Grave (if a champion) also provide a soul. Wukong's clone does NOT provide a soul. Illaoi's spirits also provide a soul if they ARE killed. As an Illaoi main, I'd personally quite like that to stay, but it probably shouldn't for obvious reasons.
How can we test the issue?
Do the stuff above. ^
Happening 100% of the time, at least in my testing.
All of that said, I'm loving the new Dark Harvest, but it should really have a cooldown indicator.