As far as I know, this is more like a setting that needs to be turned off rather than an issue. You will have to go to Settings > Call Settings and locate the option that automatically locks your screen while you are in a call. My Samsung doesn't have this, probably because I have the latest version of Android OS, but go ahead and check it. I've also searched the web and other users recommended this solution as well.
You can also access Call Settings from the Phone app. While in the Phone app, go to its Settings then you can configure it as needed.
