How much peat, and what type ?
Bog peat can be highly acid, Coir peat is less acid.
Using a high % of bog peat in the mix can cause pH problems unless Lime/Dolomite is added to adjust pH.
If the pH is wrong, all trace elements will be out of whack, including over availability causing toxicity.
Too much peat and your tree is basically planted in a bog marsh.
The acidic peat also attacks roots. The acidic peat also attacks roots.
Not sure if that is your problem , but it looks like some of my mix mistakes ??