m0useanswered grow question 3y ago Some plants just get that when when exposed to LED's and if its in their genetics.
Their are deficiencies that are associated with phosphorus and red/purple stems but they tend to also be showing on the leafs.
Your plants are looking Healthy and I would not worry about this at all.
My G13 Blue Cindy is also doing this under my grow and she is looking healthy as well.
Best Of Luck!