I would consider myself an intermediate to advance SW user and I came across this problem and didnt know anyone else was having it. I have sheet metal profile as shown below. I made 2 cuts at an angle as shown and the flat comes out with the cut near the radius as shown which I caught and it is wrong. It should be a shape close to that of an ellipse instead of a line as shown on the bottom picture. I found out that if you do the unfold feature instead of flatten (which is what the flat pattern normally uses), it comes out right. Then I have to make a separate config that turns on that feature to make my own flat pattern config. It does this on 2017 and 2018.
If anyone has any other suggestions, let me know. Otherwise this is to warn others of this error and a possible work around.