This behavior is being investigated by Engineering. In the meantime, there is a workaround.
Workaround
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For any object where this "Mouse Up" behavior is a problem, include a
"Mouse Leave" event with a "Go to itself" link. This should prevent the Mouse Up event from processing when the mouse is released outside of the hot spot. However, this workaround may affect the playback of sounds and movies any time the pointer leaves that hot spot.