Niagara 2d fluid sim - strange artifact

HI, I am using a modification of the 2d cascade from the content examples project to make some fountains.

I can modify it kind of ok and learning, but I cannot work around this:

Those accumulations of particles along something that looks like some kind of bounding box. Is that little horizontal line that you can see. It varies in size according to the angle and the shading.
It happens in every instance at the same distance, but I cant find the settings related to that particular issue.

The simulations seems to work ok apart from that accumulation. But in the final cinematic it looks strange, and I prefer not to fix it in post.

If anyone can help me I would be grateful.

Thanks
Matias