Coir has some odd calcium uptake properties- It'll bind Ca molecules until it reaches saturation. Coco specific nutes just allow for this in their ratios, or utilize a " buffering agent" that saturates the coir w/ Ca prior to planting.

Because the media skews the available Ca to Mg ratios, you start seeing odd micro def/toxicity issues.

Not something I've read- something I've seen, in other gardens using CocoGro. I think that you just have some cultivars which are more sensitive.

I don't think that you have a general overfert going on- Your leaf tips don't have the characteristic burn.

Anyway, google " calcium cation coco coir" and " blossom end rot" to get some info/ pics of what I'm talking about.

Also, it's important to never let coir completely dry out, as this can cause weird lockouts.

Anyway- my $.02US