Magnesium deficiency like you said... or maybe a nutrient burn as you are using the Hydroponic dosage , in soil you have to use half strength so following that chart you are giving x2 nutrients while most of the growers are using half of the classic schedule and they slowly increase the dosage until reaching full strength. Check your Ec/Ppm level , it could be the key . It could also be a nutrient lockout caused by a too heavy nutrients dose so maybe a flush or just water for the 2 next watering will sort that.