Some thoughts on the seedling problems.
I’m more or less applying some general gardening concepts to this problem.
Poor root development preventing proper feeding.
I had a client with some apple trees incorrectly planted, they appeared sick and had the crusty browns. and looked wilted. When I pulled the trees out they had literally no root development outside of the burlap bag and this was due to the hole not properly shaped and sized and a concept called “texture interface”.roots just did not want to move out of the bag into the hard soil. The proper hole was supposed to be dish shaped properly sized and the backfill was supposed to be new soil free of rocks.
So… last night I pulled one of my mothers, just had a lack of vigor, And I had a similar problem, when I potted it I put a layer of really wet coco and topped it off with some dryer stuff, well, same problem… The lower layer compressed some how and roots were not going into it.
The mother was originally in 1 gallon rockwool cubes, and I think this also prevented the rootball from poking out, I never stripped the ball because of ball damage.I thought it would be ok. this is somewhat similar to your seedling problem, off color stems, lack of vigor, and what looks like a combo of feed problems.
Just some thoughts on this…