Some kind of deficiency?

It's not a guess. your soil has sufficient nutrients for cannabis to grow.. By adding salts to the top they don't get absorbed you simply don't have enough root mass to intake the nutrients you are adding... Hence you make your balanced soil mixture "hot"... I looked over your diary.. I know what I'm talking about.. overfeeding doesn't have to have burnt tips that's nonsense.. Your dark foliage is what says hey there is too much and it's simply science that you have over saturated the medium because of your addition of nutrient before the root mass has sufficiently developed and the plant size is adequate.. You should not really feed cannabis in soil for weeks 5-6, growing in coco and hydroponics is different... I am 6 weeks and it's only been a very light mix of worm casting tea and they have taken off in the last weed because there is sufficient root mass to intake the nutrients I am adding.. This should makes perfect sense to you. if not I can explain it further..