rhodes68answered grow question 5 years ago I would have to go back and ask how you preped that coco.
That is the biggest source of issues in that medium, improperly prepared, not washed well enough, or not buffered correctly.
Coco does not technically need perlite but it sure makes thing work better, 20-40% is the range I prefer 25%.
Now yours seems that the coco was not buffered correctly. Could use some perlite but thats not your issue.
What happens is the coco begins pulling calcium away from the plant and that is a calcium issue you have friend.
Some strains will react to this situation worse than others, high need of calcium or magnesium.
Giving that plant more is about all you can do your PH is fine 5.8-6.0 are perfect.
And in coco you feed from the start of those true leaves. Gently but you feed.
Hope it works out 😀