Welcome to my life No, probably not nutrient burn, but I also can't definitely tell you what it is because I've been having problems like this for months. My working theory is unhealthy root system due to slight-but-persistent overwatering.
Drama queen One of my girls in flower has zero fan leaves. Zero. Noticed yesterday she's throwing out a fresh batch of white hairs because the best time for a growth spurt is 64 days into 12/12 with no fan leaves.
Weed is a weed. Yeah, some strains are finicky and annoying to deal with, but at its core, weed is a fast-growing annual that will survive in almost any conditions, even if it doesn't thrive. I once stumbled across a weed plant, maybe a foot tall, growing in a pile of leaves/garbage in the gutter alongside a 6-lane street. Someone threw their stems and seeds out the window, and this versatile little survivor germinated and was growing just fine. Wish I still had the pictures.
Anyway, my point is: even with problems, and even with a lot of problems, your plants will probably survive, and you will even get some good smoke off of them. You're learning (and so am I ). If you're not desperate for buds or afraid of failure, turn this into a legit experiment: have a feeding/water gradient where one lucky victim gets all the food/water you want to give it, the opposite extreme gets little-to-nothing, and a gradient for the rest. You should hopefully have observable differences based on different inputs, which might help you identify the best path forward.
... or at least that's what I want to do. Trying to get a vegging girl healthy enough for several clones, at which point I'm busting out a lab notebook to figure out why my green thumb turned black.