So, it could be a systemic infection by a fungus that was on the nut, but since you treated with two fungicides and still have the problem AND the damage is appearing from the edge and moving into the center then a suggestion to consider is that you have a physiological stress.
As you mentioned you the roots were bound which indicates the plant is stressed. The recommendation is to put them in a bigger pot with well drained soil
"If the taproot has become twisted at the base of the container, it should be
straightened or cut to encourage new taproot growth." From University of Georgia, https://secure.caes.uga.edu/extension...
Here are some good tips in the University of Florida http://edis.ifas.ufl.edu/hs229#FOOTNO...
When are you planning on putting these outside?
Is it single plant? What do the others look like?