Not sure if bug or I just don't follow as it usually is but how to get Tree Huggers Steam achievement as Unfallen? The descrition says "As the Unfallen, extend Vines to 5 systems of an empire with whom you have signed a Peace agreement". Also it has a counter, 0/5 in my case.

So I've sent Vineship to the system of my AI ally, entvined it and... counter left at 0/5 mark. Then I changed relationship with another AI to Peace (signed Peace agreement?) and entvined one of his systems and still counter shows 0/5.

What I'm doing wrong?

---UPD---

Nubmer changed to 1/5 somehow. Does it count entvining one system per faction with Peace or what? Anyway need an explanation or this thing.