Because your problem is at the top of the plant and going down and you say that you only feed 1/2 strength.
You have a deficiency. They show a def in all macro nutrients!
When I run a synthetic, I run Hesi and have for a long time.
A few Hesi Suggestions.
In veg.
Use as directed EXCEPT, only use the ROOT 1-2 times a week as they have changed the formula and "failed" to inform the general public to cut back.
If using RO be sure to add a CalMg product.
If your vegging over 4 weeks. Do a wash and continue - keeps salt build up down.
Wash at change-over to flower!!!!! Add 3ml of Phos+ at change over watering and only do it ONCE (helps kick flowering hormones into gear).
In Flower
Use as directed !
Wash at the end of 3 weeks before you start adding the Phos+!
Increase the Phos+ to 15ml for the LAST 2 WEEKS before wash and add an unsulfured molasses if you like (5-10ml per gallon of nute solution, an "old school" trick).
WASH for 7 DAYS!
Good Luck