m0useanswered grow question 3 years ago I see the discoloration your talking about but I don't think that's Nitrogen.
It does look a bit like magnesium or a PH flux/Root issue.
Its hard to tell in the early stages they look the same.
Are you feeding CalMg at all?
If you using Liquid nutes the PH is to be a bit lower I think. 5.5-6.2?
Could try adding calmg and lowering PH a bit to see if it resolves,
Best of luck