Is this a nutrient deficiency?

I am not an experienced enough grower to be able to provide any firsthand advice, but I have used this site to troubleshoot with good results: https://drcannabis.io/symptoms - good luck!

Checking Ph of runoff in soil,is usseles,you have to check it in soil,with special ph meter for soil,this will be more accurate. About Ec i would say ,yeah they are nutrient deficient!

Personally I wouldn't worry about ph in runoff. The EC measures the run off, you need the correct pH BEFORE u feed or the plant can't uptake any nutrients. Ph of feed water needs to fluctuate between 6.5 and 7.5. Coco is 5.5 - 6.5....some people do confuse coco with soil though . Red stems defo are phosphorus deficiency but your plant is lacking everything at the minute. Adjust feed ph and possibly increase nutes, I know coco ec should be 1.6-1.8 at your stage, not 100% sure what it should be in soil.

Locked nutes, your answear is in your diary bad PH. Manage PH to correct range. Nitrogen, Potassium, possibly Manganese are locked. You must manage PH first.