Hey tick tock,
So anytime i got those rusty splotches it ended up being lack of a calmag supplement or i had a ph problem once that reeked havoc. Since your ph # is good, unless you want to recalibrate just to check, look into a calcium/phosphorus issue. Maybe drop your bio grow and pick up the bio bizz calmag supplement and the problem will probably sort itself out. Unfortunately the damage is usual permanent but you can stop the spread and make it through flowering. You could possibly up the bloom nutrient for a higher phosphorus level since purple is often caused by that...but being in the mix already I'd lean more towards a small calmag dose each week.
Happy harvesting. ✌️