Using the Particles.SubImageIndex has no effect on what portions of the a UV are rendered in Niagara.
Tested in 4.21.2 (CL - 4753647), 4.22.1 (CL - 6063286), 4.23 (CL - 6533064)
Expected: The SubUV would change its image based on the index provided
Result: The SubUV does not change
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-74706 in the post.