Most common cause of this type of feathering is the K slow feathering gene. It's not that uncommon in poultry stocks and is used in show stock barred rocks because the slow feather growth helps with much crispier barring.
While they feather out slow as chicks, as adults they have normal feather cover. That one sounds a little extreme but still in the range of normal K feathering rates...
but- K has no effect on body weight and growth, so it being tiny is somewhat a concern.. perhaps genetically inclined to be small plus has K...?
there are other genes that cause very late feathering but they are uncommon so for now, they are not likely to be cause unless proven otherwise- defective feathers as an adult for example.
