Question Flickering fade shader

Hi all. This should be a general shader question but i'm using ShaderGraph in case it's related

Basically i'm trying to recreate this:

So i'll have particles spread, with infinite lifetime, as the "path". Not rendered.
The trails that follows will use this shader. Using alpha clip to determine the final "length" of the trail from the base, with a 0-1 alpha "Color over Trail", so that all each particle needs to do is change its Start Color's alpha.



So far so good.

But this is happening. Here the particle (and the following trail) is going upwards slowly and start color is full, not changing

Reducing the StartColor alpha will shorten the trail, as expected, and the flickering is only "less intense".
Notice also the flickering gets more intense the longer the trail is stretched


Here there's a lot of "Clamps", "Maximum" i added because i thought it's a division by zero issue. But it doesn't help

In pseudocode it's something like this:

Code (CSharp):

1. float vertAlpha;
2. float uv;
3.  
4. float fadeValue = vertAlpha / uv.x;
5. float fadeFlip = 1-fadeValue;
6. float valueAtUv = uv.x - fadeFlip;
7. float fraction = valueAtUv / fadeValue;
8. float fadeOverTrailAfterClip = Lerp(0, 1, fraction);

And the flickering happens around when UV.x is 0, vertAlpha is 0; So, around 0/0 in line 4

Any help is appreciated,
Cheers