You are admin, click ban button in connections to ban someone, then ban duration dialog appear, meantime user disconnect, you click ok. And since there isn't any kind of message to confirm ban, you probably think ban is applied, while in reality its not

Tried same on ingame server and by doing same you will get following error: ADMIN BAN : Could not find client for connection 0

Well,

That applies to the kick button too, quite logical tbh.

Thats why its better to use /ban

Applies online or offline. Not sure if this can be called a Bug at all..

As it throws an error message, that means it's handled, thus not a bug. /ban command is a useful thing and makes no mistakes.