What is color leakage? Why is it bad?

[Rare]

Hard to say, but I think that is the pullet. The feathers are round in shape.

His tail looks like first picture's

 

[NagemTX]

Here's the pullet with leakage around her neck.

 

[JacinLarkwell]

Here's the pullet with leakage around her neck.

View attachment 3628415

I have a pullet with that leakage too. She's too elusive for pictures though