Storyline 360 issue: Base layer automatically plays after preview of any layer on the slide.

Hi,  I make a lot of use of the layers on slides.  Since upgrading to 360, every time I preview an individual layer, the "preview" automatically plays the base layer of the slide.  This only happens when I preview a single layer.  Everything works and looks great when I preview the slide.  Two questions:

What am I doing wrong?

How do I make that stop?  

Thanks!