écran de chargement vidéo non souhaité

Bonjour et bienvenue, Richard.

Since you have a video on your first slide, this article will explain why you see this behavior on modern browsers. 

One option to avoid the Play button is to insert a blank slide as your first slide and add a trigger to jump to the next slide when the timeline starts.

Keep in mind that this will also be browser-dependent, and while it might show the slide without the Play button, it might still require the user to click on the screen to hear the audio. 

I hope this helps!