I had this bug for ages, and it still persists in EssentialsX.
If I ban a player and then unban the players is not removed from the banned-players.json and my only option is to edit the file manually and restart the server.

Increase your usercache

user-cache-size: 1000?

if player don't log on server (and it's kicked fon banned) you can't pandon her.
(si le client n'essai pas de se connecter on ne peut pas le pardonner)
it's a minecraft isue on 1.8 (i think it's 1.8 but 1.7 ?¿ the source is json file )

Use: /minecraft:pardon playername

not an actual issue