Simple trim with regions problem

I can’t get past something which looks like a simple trim with regions problem. At least one result seems wrong. Am I using it wrong?

I marked a problematic solution with white color.

trim with regions.gh (56.0 KB)



You won’t be able to see this error if you provide a base plane for Trim With Regions.

Thank you. I just discovered it too… Simple as that. Why this one single line is acting like that without a specified plane is very mysterious for me.