Seems like a hot mix for a young plant based off your first post.
The plant I posted is in $4 bunnings seedling mix (Coco, peat and 1/3 perlite) and hasn't had a single nutrient added yet. The seedling mixes I find have the least nutes, and I have the best turnout with it. Every time I blend some fancy soil, I get issues.
What is the pH of your runoff slurry? I've seen plants stunt when the pH is too high as the nutes don't break down as well in alkaline environments, but then again you don't have bad discolouration or anything. The thing is, U usually see nitrogen toxicity at high pH, and it doesn't seem to be shiny and dark, and doesn't have strange leaves.
The one or two plants I did have that stunted, I did a full flush of the soil with pH balanced, filtered water, let it dry for a day and treated the medium with Cannazym and rhizo for first week after, and then back to feeding. They were still in mid veg though so, and was probably a completely different issue.