I would not say this is a deficient type situation... My ultra Violet Og is getting plenty of P and K and it's doing the exact same thing.. Started about 3 weeks into flower. I know it's because of the Ultra Violet OG Genes. I've got 6 different strains on the go right now so when I see purple stem I will usually add like an Extra 1/2 feeding of FloraBloom (I'm hydro too so I just add some P and K to the top up). And it will stop it in it's tracks. If it is indeed deficient. And it will stop spreading to others leaves and the ones are purple will start purple. If by adding the Extra 1/2 amount the purpling still continues like it has with the Ultra Violet Og, it's from the Genes. I hope this helps and happy growing.