From what I'm aware of, /topic expects arguments. Triggering /topic without arguments might trigger the topic to be shown instead of set.
A possible fix might be changing the lines 131 and 132 of /js/irc/commands.js to the following:
cmd_TOPIC: [true, 1, 0, function(args) {
this.send("TOPIC " + this.getActiveWindow().name + (args?" :" + args[0]:""));