I’ve had the same method for the scratch pad and worked fine. So I think the problem may exist outside there.
Firstly, are you using the “Niagara Set Position Array” Node in the blueprint? if so, the input TArray must have the same number of elements as the particles in the emitter (if with that scratch pad implementation).
It seems you utilize “Spawn Rate” and it may increase number of the particle through a cycle. Please make sure the number of elements is in sync with the blueprint and the emitter.