Hi c3g
Thank you for sharing your node and making this tutorial. It's beautiful. :)
We should have more tutorials like this for nodeeditor.

I checked your node and had the same problem you mentioned. (Unpredictable result on every run)
So I tried to fix it.

In the node file you attached, there are some "trimObj" and "join" nodes that I think
they are the reason of changing the indexes on every run.

Moi will make new geometry every time you change the graph.
Some factories like fillet, trim and join can change the geometry indexes and their order.
So, if you are using these nodes, it's better to avoid using the "idxselect" node when possible.

I've attached my modifications on your node.
Also extracted the profile curve's points, so no need to select anything before starting the node.
And finally made the leaves a bit wider and taller. (Just for filling the free space)

Thank you again, And keep making cool things. :)