How was the coir rinsed, cleaned and buffered? That's way to big of a pot to start in coir. You should not treat coir like peat or other soil like mediums with dry backs between waterings.
In the beginning you have to feed them by spray bottle, not to wet, not to dry til their roots are established. Then feed daily with 20% runoff.
I suggest getting 100% coco peat and starting in a way smaller pot until it rooted up nicely.
Also for seedlings or even clones bottom feeding is the way to go.
You mist when they are not quite out yet or are slightly popping up.
Then once they are out and have their first leaf apart from the cotelydons you bottom feed.
Start off straight away with 0.6EC at the least.
If your coco isn't buffered and you use R/O water add calmag.
Always have clean coco or wash it...
Which is not a nice process.
After bottom feeding a few days you can start with regular fertigation from the top.
Also dry back cycles especially in veg are needed. Don't ever dry out the plant.
But going back a good 20-30% vwc
( so from about 50-55% in pure coco )
And eventhough it might seem scary it causes a slight stress response needed for the plant to root out more.
Ps: I fully agree don't mess with autos lol.
So disappointing bud always