Why do leaves get yellow and die?

looks like normal, because the are not getting direct light (: would not worry! adjust ph to 6.4 and feed some N 🌱💚

Hello mate, you have an issue with Ca/Mg caused by a pH fluctuation or a misfeeding of Calmag product. The first cause is often link to the rate of Ca/Mg in the water used to watering the plant, if you use organic feeding you must have the rigth rate of these nutrient that is 3/1 or 4/1. Good luck and happy journey!

Yes, I believe she's hungry and resorting to starting to use up her mobile nutrients stored in the leaf tissue. This is normal and slightly early to see it but not unheard of. A little liquid kelp is good in bloom and may help boost some n too. They still need some N during flower just don't give crazy amounts to avoid grassy flavor. or any all-purpose bloom fert should have some N as well in the right ratio. if not in flower I would say feeed some fish hydroylasate.

Looks like she didn't have enough nitrogen going into flower, usually the lower leaves will do this. Your using nutes now so it's probably something lacking a few weeks back.