There is no need to stretch the loop after you have cut the end off to remove it. It does not matter if this leaves gaps between objects on the track unless of course you want the next audio object to start immediately after the cut.
As well, stretching an object, like a soundpool loop by dragging the end causes it to repeat. That is, if you have a 2 bar loop, stretching it to 4 bars will simply repeat the first 2 bars as bars 3 and 4, etc.
A good way to see how Magix handles loops, open and inspect some of the demos.
I see. Off the top of my head, I suggest trimming out the part that you don't want and then copying the trimmed object to the new location. Hold down Ctrl and drag the object with the left mouse button to create a copy. You will want to drop the copy at an even bar or part of a bar, like at a quarter, sixteenth, etc.
As you saw, dragging the end out just puts back the original part of the loop.