Pro-mix organic seed starting blend is what I used for years, it works awesome. Lately i just use pro-mix or proterra with some organic 4-4-4 mixed in, same results.
It's more than a hot spot when you are cutting the medium by 25% you are reducing it's nutrient content by 25% mixing it would do nothing to reduce "hot spots" it's adding perlite that is lowering the "heat" of the soil. Just try putting cuttings and seedling directly into OF after mixing it well and not adding perlite.
They'll have issues and then you'd see it isn't a homogenization issue in processing the material (hot spots) it just is over amended for young plants.
