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I'm not sure why those lines show up. Maybe a software bug in CATIA?
Using Chamfer to taper the front and rear faces is an interesting technique. Chamfers are normally used to eliminate a sharp corner, like a fillet. They normally are short and don't extend the entire length of a face, like you've done. But there's nothing wrong with using a Chamfer for the tapered faces. Maybe it was confused with the slot in the middle?
You didn't show the tree diagram, so I have no idea what features were used to model what you have so far. And there are many ways to model a part like this.
But I suggest replacing the Chamfer with a Sketch and use a Pocket to cut-off the faces, like the picture below.