Why does the delete key increase the opacity of an ellipse?

Hi,

I have an App Designer GUI that draws elliptical ROIS on images. I just made a callback that deletes the currently selected ROI by pressing the 'delete' key. The function works fine, but if 'delete' is pressed more than once, the remaining ellipses' opacity increases with each press.

Why is this happening, and is there any way to prevent this?

Thanks