Probably because that's not a closed profile (assuming I understand the question). Add a line where shown below and try again to see if that gets the result you want.
You can't do this one? Could you tell us what do you want to make with this one?
Please add some more text to your post.
Explain what the goal is and what the problem is.
To me, it's a thin wall extrude that's behaving normally.
If that's not what's required, what is?
In addition to the missing line -
note the missing Tangent Relation.
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