A user joins the channel #help and is idling. After that one of the helpers or i join the channel too. In our nicklist we can't see this person, but he is on the channel and talk with us.
This problem is not only with the same person and this happend with different versions of mirc/xchat too.
Ill agree with Grunt here. Currently there is no user mode to hide someone from channel nick lists so it almost certainly has to be either a client problem or channel mode +u