Isn't ProMix in fact peat?? I think that's the issue, you're treating a peat based medium like it's soil, perhaps? Not a bad way to grow nor is it "wrong" but I just think the plant is having a slight reaction to how you're growing it.
Overall, the plant looks REALLY nice. Don't beat yourself up over a couple spotted leaves, especially on such a large plant. Prune it back a bit, as long as the spotted leaves aren't widespread throughout the plant, I literally wouldn't even worry about it.