Hello Philippe,
Happy to help! Thanks for sharing the video. It made me understand the behavior better.
To determine if this is a bug in the product, I tried adding entrance and exit fade animations to objects in the attached Storyline project but did not see the glitch. I used the 64-bit version with the latest update, Build 3.89.32618.0. Did you notice this behavior during preview or in a published output? I published my project to Review 360 with similar results.
I'd appreciate it if you could share your Storyline project containing the affected slides with us here. I'd like to see whether some triggers or settings may be causing this glitch. If you prefer to share it privately, you may open a support case and attach it there. We'll delete our copy of your course when we're done troubleshooting.