Autoflower believed AK47 can’t identify if a deficiency or abundance, or type of problem
Id guess magnesium or it was overwatered/ had a little root rot… but who really knows. That is why in living soil organics we load the bank and keep it loaded rather than chase problems. Simplify the growing process… Bigger containers with living soil. Feed the soil and the microbes. Your plants will uptake what they want and leave the rest. When they are done, they’ll stop eating with a full bank of food and just fade away and finish.
The other problem with small pots (Especially hard plastic - much easier to root rot than when in bags) is they will be over watered/ under watered everyday even if you do it perfectly which will certainly affect maximum potential. Check the density of the soil and make sure it has adequate drainage. I definitely prefer bigger pumice rather than small perlite that floats to the top. Even Promix can lack drainage from adding worm castings without additional pumice/perlite/etc.