I've farmed a second one, so I have 3 in total. The one that the story gives you is in a place that you know, but haven't had the ability to open until this latest update. The story/Evie doesn't send you to find it, so you'd have to rely on your own curiosity/memory.
There are indeed 10 philweed plants to deposit. They've been increasingly obscured, to the point where I've stumbled onto the last two. I'm going to speculate that you've missed the one in the latest update. This one is in Casula temple, second room, lower left, obscured by the palm tree.