I was going to suggest a P def, i see you already are considering it.
The Red stems and the spots started on mine. It was PH that was off but it started the same way. However, I also had a nitro def due to the same ph issue so it's difficult to compare.
Can you confirm your getting a good PH with run off and if u are, are u light on the P by any chance? 6 days into flowering and then you started getting this, it also points to a flowering related nute. Which fits the P theory.