Hanging Ivy sometimes works and sometimes you just see the brown vines as opposed to the greenery.
This does not happen in all locations. Seems to happen more in the highland.
I can second this. Had the same experience on both official and private server (no mods), also in the highlands, and after logging in the next day. Leaves were gone with the wind.
First I thought it became autumn, but then I remembered there are no seasons in Conan Exiles! 
If you remove them and put them back on the green comes back.
Not sure if this will help. But I took some of the Ivy down and then put it back up which refreshed it. It then rained and when loading back in the green was gone again
Ah interesting. I thought it was because of the rain too.
To try it, we put some on our jungle base while it rained and the next time we came back to the base (it loaded again) the leaves were gone.
I’m on PlayStation and I’ve seen this on single player/co-op a lot, but it seems to be intact on official… so far anyway. 
Rain kills plants!!!
Ivy appears to be lush in the highlands again.