What's happening? Need someone with more experience

Calmag das Problem lösen. Könnte aber auch sein dass zu viel gegossen wurde und nicht vergessen nur jedes zweite dritte Mal mit nutrien gießen. Welchen pH-Wert benutzt du es könnte auch am falschen pH-Wert liegen. Und nicht vergessen Lieber ein bisschen zu wenig Dünger als ein bisschen zu viel und eine tote Pflanze. Hallo ich würde die mit dem pH-Wert von 5,9 bis 6,1 gießen alle zwei drei Tage

Zinc deficiency. A dose or three of soluble trace element/trace mineral mix, available from any decent harden centre, should solve this. If it was nute burn, many leaves all over the plant would be affected, not just a few top ones. As it is only at the top, this indicates that it is a problem with immobile nutrients, one of which is zinc.

Looks like your feed was too strong and you have some nute burn. I would just use plain water for the next few days and then try again, but stretch what you put in the 0.5l into 1l. Sometimes some water evaporates and the plant drinks more water and not as much of the nutrient, you now have similar amount of nutrients you added, but much less water making your feed much more concentrated. I prefer to go very low on feed, but add food to every water. It's safer than dumping all the food for a week or 2 and then as you soil dries the ratio can get way too high.

Water demand gets Higher in pre flower. I dont know ur Pot and plant size. But 0,5 l every two days seems too low