What's the temp range in your grow space? I've seen stems change like that when the temp in the evening gets down in the 60s range.
It's at a warm 73-80 all the time, mostly 77-79....55-60%rh...(6)37w cree cxb 3950 at 28".... .8 ml/L sensi a/b, got my ph to 6.4 for the promix. The double grape above is in promix. The Other two plants in coco are on .125 teaspoon of veg + bloom powder and ph of 6.0. Ppm between around 300. They are all showing purple stems and veins in the leaf