Red Stems…

I find my stems go red, sometimes Mg helps sometimes P helps. its weird. I also feel my lights cause some of it. I had one plant not do it and one do, same seeds both looked healthy. I could be fucking up in my nutrient profile but they still do ok, this looks a lot like my plants. I also find the growth closer to the light is more red vs the older stuff. it can get so saturated it turns more purple/red. I also made a question about it. growdiaries.com/grow-questions/69162-look-at-this-pigmentation If the plant is not showing any other signs of P deficiency like leaf death, then I'd leave it as is. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg Good Luck!

Are these stems and veins in the leaves the only "problem" you're encountering? I think you're overthinking things. Acquiring a little color to these areas is pretty common, especially in older growth, and isn't necessarily a sign of deficiency.

Could be a phosphorous deficiency, but the plants seem healthy enough, I wouldn't worry too much. A P/K booster may help.