Oh, I see.
That's not a bug at all, you specified it to work that way.
Code:
on *:INPUT:#: {
  if (%multi) {
    var %f $fline(#,$1 $+ *,0,1).text
    while (%f) {
      var %n $fline(#,$1 $+ *,%f,1).text
      msg # $effect(%n) $2- 
      dec %f
    }
  }
  else { 
    var %n $fline(#,$1 $+ *,1,1).text
    msg # $effect(%n) $2-
  }
  halt
}
menu channel { 
  $iif(%multi,$style(1)) Multi user : $iif(%multi,unset %multi,set %multi 1)
}
alias -l effect return 6¤3.11·13´6`9·4.13¤ $+ 1 $1 7¤3.11·13´6`9·4.11¤1
Try this and let me know if it's what you want. You can right click in a channel and enable "Multi user" so that if you type "nill" it will message the channel for each nick it matches. like Nillen, Nillens, Nillensbot etc.

Nillens @ irc.twitch.tv
Nillen @ irc.rizon.net