Problem creating Boundary Boss/Bass feature.

Hello,

I am working on an exhaust manifold and i am facing some difficulties creating boundary feature. As you can see from the picture, I am cutting the runners and the turbo flange and want to connect them using boundary feature, so the model has better shape. No matter how deep I cut the runners, every time I get the strange creation circled in red on the second picture. Can someone help me with this and explain why this is happening and how to fix it, please?

SolidworksParts And Features