Nitrogen deficiency during 3rd week of Flower.

Should I be concerned and do something to resolve it.

Or is it just fine, let it ride, and maybe actually be improving the plant?

Growing in Coco using Blumats and a weekly ‘rinsing’.

Yes you should be concerned just as much if it was deficient in any other nutrient.

Its a MYTH marijuana needs less N in flowering. An 8 weeks flowering strains need more of everything in slow incriments until the 6th week of flowering, and them the food is only slightly reduced.

Nitrogen is to the plant as what Protein is to a human.

Humans eat protein, and turn it into Nitrogen.

Plants skip this process.

If you were trying to grow/get bigger, and stronger it makes 000 sense to cut back on Nitrogen/Protein. In the 3rd/4th weeks it needs a boost in P/K, while the base formula is also slowly increased. In weeks 5-6 they need more Magnesium.

I generally continue to feed with "veg" ratios for the first 2 weeks of flower, then start mixing the veg and flower until I am using pure "flower" nutrients for the last couple of weeks.