It could be strain related or possibly the starting of P deficiency. I'm doing Red Poison and she has purple stems without any other signs of deficiency.
I also have another stain outdoors in a mixture of bad soil and she has the P deficiency... purple stems, signs of other deficiency... cal/mag and possible iron.... the purple stems were my first sign, then the leaves started yellow spots and such...
I'm still extremely new at this so, I could be completely wrong on everything...lol
Hope everything works out! 😁