Unexplained Disappearing Objects

We are building a matching interaction in Storyline that mimics the Rise matching knowledge check.  We have been able to get everything to work correctly but when you drag one picture on top of another it sometimes disappears.  We cannot find any pattern that would explain the disappearance or a trigger that might be causing it.  We tried replacing the shapes that we used as drop zones with hot spots and had the same result.  We would really appreciate any insights anyone might have.  We are including the SL file and a GIF that shows what is happening.  Here is the a link to the SL published to Review.  Thank you in advance!!!

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