I said to you, but will advise one more time !!!
If you overfeed with nitrogen it cuts out calcium, when calcium cut out all other nutrients are struggling to be absorbed, especially magnesium, that's why they go together.
Now your soil is full of food, you were adding CalMag, but plant already shows signs that can't uptake all, so build up is forming.
Just 6.3 pH water , dry out, water, dry out, water. Only then you can return to BioGrow and most probably open bio bloom too by that time.
All the best !
@AsNoriu, got it, thank you so much!! I actually didn't give them any BioGrow until I repotted them, but when I first planted them in their nursery pots I reused some of the soil from my previous grow; like the parts of it that weren't penetrated by any roots. Now I see it where the build up is coming from: the soil I used was likely packed with all the nutrients that the previous plant couldn't absorb. I'm gonna stick to your advice, and given that now they got access to lots of fresh soil, I hope they recover soon! Thanks again for your input :)