Two slides visible at once, one faintly showing through the other. The stacking effects lay panels directly over each other by design, so a panel without a background of its own is see-through and the slide behind it shows.
The fix
Give each panel a solid background — a colour or an image — using your page builder's own Background setting. Every panel needs one; a single panel left transparent is enough to produce the effect.
Why the plugin doesn't set one for you
Backgrounds belong to you and to your builder's own controls. If templates arrived carrying a background, it would fight whatever you chose and your pick would be quietly painted over. Templates ship with no background at all so that the colour you set is the colour you get.
Using a semi-transparent colour on purpose? That works, and it will show the slide behind it — that is the setting doing what you asked. Use a fully opaque colour if you want a clean stack.