Hello friends, do you know what kind of issue coul...

Hi bro! Welcome to the “CalMag deficiency community”! When you grow in coco, the only thing to do is to always use special nutes for coco, or apply additional CalMag as recommended by manufacturer for coco substrate. It naturally locks some part of available Ca, Mg, Fe and releases Na and K. If the substrate was not properly buffered with CalMag, you need to feed more CalMag, basically raising EC until substrate gets saturated enough and most cations of Na and K are released. If you want to flush coco in veg or flowering before the flushing week (which is probably a myth), never apply just RO or tap water. It must be CalMag water. Lockout will take place until all coco tissues break down by roots and microorganisms, that means never. But it will surely be less in time. Your plant looks great, there’s not a big issue. You may try CalMag spray with pH 6.0 1/2-1/4 strength of recommended for watering. Effects of foliar spray are fast, spray from underneath. The damaged patches will not recover, pay attention to new growth. If it’s rapid, you’re good 👍