An Aura is based upon the proportions of the trigger image being used, it is the canvas of the experience.
The position of all Overlays are calculated relative to the trigger image's proportions - so a change to those proportions with mean the Overlays already added to the Aura can get stretched or squashed.
Thankfully this is easy to fix by selecting & right-clicking on an Overlay and choosing the "Restore Aspect Ratio" open, this pops the Overlay back to its original dimensions. You can then hold shift and drag the edges to re-size it while maintaining the ratio. You can also access the command from the alignment tools along the bottom of the 2D workspace - in the "Resize" menu.
One way that you can avoid this entirely is to crop or add empty space to a new trigger image so that it matches the proportions of the existing image. That way no ratio change has occurred.