I see the problem, but I am not sure what causes this. I mean this is probably caused by the fact that some frames might have more than one physics update or none, but still I am not sure at what point it happens.
VFX does not update in any magic asynchronous way.
First of all check graph asset settings, you can set what kind of update you want - this might actually solve the problem.
Another candidate you are using probably spawn over distance or time in Spawn context. I am not sure, but possibly this is might be the cause of the problem (somehow?), try constant spawner and see if the problem persists.
Or distribution of particles along the line. If update options does not help, try to locate what exactly causes the problem. Also you might find useful one of these links: