I'm working on a low poly animation, in which there is a bottle of water and a tank of petrol that slowly empty out.
I figured out that, in order to do this, I'd first render a picture of the plastic bottle and a video of the water going down separately, and eventually put them together with some blending option on After Effects.
For the animation of the liquid, I've used the same solid from the bottle or tank, and made it slowly disappear intersecting it with a big cube that moves down and a boole effect.
The problem is that, when I render it out, sometimes the top of the liquid, which is actually the bottom of the cube, flashes and disappears for a few frames, making the animation unusable. Here's a video showing two seconds of the animation and then a single frame of the whole problem: