Don't abruptly end the boxing added plate with vertical end especially in or near a high stress area nor should you weld in a highly stressed area. That's why you taper both ends of an added plate that forms the box. A frame rail must flex a little rather than fracture or propagate a crack. Abruptly ending the boxed area simply chases the solid non-flexing area to where the frame's now weak point will now be. If you don't realize or know where the high stress areas are, go to someone who actually does. I hope that helps you to understand better.
Few cowboy welding shops will know either but will tell you about the thousands of frames they've welded before if you believe them.