Problems saving a group as a picture

I'm trying to create a mask object that will hide the content behind it except for a transparent window in the mask.  I've created a rectangle the same size as the slide and same fill color as the slide background.  I then add a circle of a different color and group the rectangle and the circle with the circle on top.  Then I save the group as a picture named mask.png.  Then I delete the group and insert the picture mask.png but the circle isn't there, just the rectangle.  My next step would be to make the color in the circle transparent to complete the mask.  I've done this successfully before but the problem is the inconsistency in saving the group as a picture.  Sometimes when I save the group both items are present in the saved picture but more often than not only one of the items in the group is present in the picture.

Has anyone else experienced this problem with saving grouped items as a picture?  I'm working in Storyline 360.

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