As freenode is currently quite heavily attacked by spam bots, the #haiku channel can only be joined when your nickname is registered. Otherwise you’ll be diverted to #haiku-forwarding.
If you use Vision’s “Automatically execute” to identify your nickname with NickServ and join channels, the joining of #haiku may be quicker than the identification at NickServ. In that case, just
/join #haiku manually after NickServ responded.