Help here please

There hungry , double their feed and add cal mag before adding nutrients, this will help stablise your ph if you using reverse osmosis water. Dm me if your unsure ✌️

Its pulling the nutrients from the fan leaves and putting them into the flowers to bulk them up and allow more light penetration. Plants are smart. Up the bloom nutrients

My first question is why you switched to 12/12?? Autos don't need that switch and won't grow well under 12/12 ... ideal is 18/6... I normally grow my autos under 20/4 just because I want them to have as much light as possible while still giving them a bit of a rest period - and some people even grow under 24/0 but I wouldn't recommend it (and I've tried it!) Without a diary here, it's hard to figure out exactly what is going on but if, as you say, you're being very light with your nutes, there are probably numerous deficiencies... there is probably an issue with your pH as well - and maybe even with the amount of water you're giving them (too little).... I don't know what "osmosi water" is but it sounds suspiciously like RO water which is stripped of every single mineral water normally contains ... when using RO water, you have to add those minerals back in to make it good for anything.... but particularly calcium! Start a diary for this grow - even if you can only reconstruct a week or two.... it will help us help you! Good luck

The purple on the leaf stems as well as the yellow to brown dyeing leaves on the lower part of the canopy can only mean a phosphorus deficiency. Up your intake on your nutes slowly when you water by 1/4. And i can deduce this being so seeing that you are in early flower. The plant will take from the lower leaves first. Leaves that aren't being used. Also i would check your roots to ensure you dont have root rot. There are some nutes that have good anti root rot microbes. I am guessing you also just cranked the light up as well. This caused the huge demand for the extra P. Good luck!