Thank you for your answer Kyatric.
I tried this :
The boolean LevelStarting is true by default and after some time, it is deactivated. And it's true that thanks to this, the animations of the blocking_props instances are playing since the first frame. Then, instances which are not on the screen are stopping their animation, great !
But as soon as another instance is on the screen, this instance doesn't play its animation, only the first one is playing.
On this screenshot, the first terrarium is playing its animation but not the second one. The third one is out of the screen so it's not playing the animation, it's OK for this one.
And something that I don't understand is that on the second terrarium, you can notice that "is playing" is "true" ("vrai" in french, sorry I forgot to translate it on the screenshot) but in fact it is not the case, the animation is not playing.
I tried to go to the third instance on the right and it's not playing its animation neither as long as there is more than one instance on the screen. As soon as there is only one instance, this instance is playing its animation.
I hope I was clear here. :)