Have you got 3 x P450 on seedlings at 24 inches for first ten days?
If so, I suspect that is the source of your stunted heights.
I would try ONE P450 for the first couple of weeks personally. Vegging doesn't take much light.
I have seen countless LED grows with very very short plants. Depends what you're after, but that amount of LED light at that height from day 1 will definitely have an effect on your plant heights and node spacing IMO.
Also pretty easy to "stunt" autos with overwatering at a young age? Soggy roots won't bother to spread out to look for water.