If you edit the part while in the assembly and the Sketch part shown, my guess is that the sketch vertex is snapping to the external part which would cause a circular rebuild that doesn't go away.. You probably need to hide or suppress the Sketch part when you edit..
Actually, the sketch of the round part and the round part itself is done completely out of the assembly and has no relations at all. The only relation was the mentioned mate between on of the points in the sketch with the origin of the part. But even if i release this mate/relation I still get the circular reference behaviour with a never dissapearing update symbol.
Perhaps this is a bug only in 2017SP1? Unfortunately this is the only version I have installed right now.