3D FlipBook - Grey flickering shadow

Hello

As I use the buttons of my mouse to open a new page instead of using the arrows, i get an unexpected flickering greay shadow. It only disappears after using the arrows again. Example my Flipbook at: https://www.archeologiegorinchem.com/brochure-onderzoek-bagijnenwalstraat-en-lindeboom/

Thanks a lot in advance Martin