I was able to replicate this, well, by accident anyways.
A long time after finishing the quest, I had a family request maximum strangeness, followed by normality. The ensuing result was normalcy, with the plants everywhere. 
Wow, didn't realize this was a bug :D Well, you still have "strange fruits" and you can plant them or get other sims infected by them, so plants still growing (kinda, they grow slowly and usually start growing anew after each reload) is not a bug. But I agree the cloud shouldn't be there.
Why this is marked as "solved"? Because it's not.
