99% of the time its due to the lack of nitrogen, alot of people tend to think it's magnesium but mj gobble tons of nitrogen during flowering... How do you think the plant is gonna produce all the new growth in flowering. It's eating stored N from the fan leaves
Looks like you topped it, I have topped too early in flowering before and that's what they ended up looking like... OR you have a light leak and they are reveggin
I can tell you from experience that 400 is waaaaaay tooo low, i'm not saying your wrong but I have fed around that rate before and had severe underfeeding symts
I have been using advanced nutes for around 5 years and I can tell you it's some good shit, you might have just started using bloom food too early into flowering
Problem is that your food doesn't have any N in it, and contrary to what you might have heard..... MARIJUANA GOBBLES A SHIT LOAD OF NITROGEN DURING FLOWERING.. I'm willing to bet that it started to yellow around then 2-4 week of flowering??