Bug: unwanted mitre at corner

See the black corner opposite the red arrow in the attached mitres.cdr.

Why is this corner mitred in X6.1, instead of rounded? As the screenshot shows, the outline pen is round.

I had this in several places (but mysteriously, other places in the original design had rounded corners as expected). I've fixed it by changing the design and adding some extra nodes, but it seems that the pen shape is not consistently working as intended.

I presume its a bug.

mitres.cdr