This is definitely going to be calcium-related, but the thing is most of the time with this it's a lockout and the plant isn't getting it all. Yes giving the plant Cal-Mag when you regularly feed is necessary cannabis plants tend to be very calcium hungry. I noticed your ph level was 5.8, In hydro, calcium is best absorbed by the roots in the 6.2 – 6.5 pH range in hydro, it’s generally recommended to keep the pH between 5.5 – 6.5, but calcium specifically tends to be best absorbed above 6.2, so this could be the cause for the lockout.