2.79b Blender texture paint stroke oddity https://i.imgur.com/fooqOYA.png
Strokes have these "bounding box" or "update area" like discolorations. They are cleared once I Undo, and so remove one of the strokes. Only in UV/Image Editor, does not appear in 3D view.

Edit: It was something with the file itself it seems, for when I copied my work onto startup file those Discolorations (they are only visual, not color information) in UV/image editor did not appear anymore.
Its not the first time that I run into it, will try to be more conscious of when it happens and update here.
Suggestion is to have a fresh start.
Delete the Brush in question and create a new one.  Use the new Brush and see if the problem persists.

You may have changed a setting unintentionally. There are many settings in Blender that you or I or some others may not have the time to track down at the moment. In the sense of triage you may have more important things to occupy your concerns.