A search on "utf" and "topic" yielded no messages, so I will submit this with the possibility of this being a duplicate of a previously reported bug.
There seems to be a bug with updating the window-title of channels when the topic is changed, and UTF8 is involved. And example of this can be seen in:
The first /topic command sets the topic properly, but the second /topic command leads to a corruption of the channel window title.
I cannot quite figure out why this happens, as not every sequence seems to trigger this. For instance, repeating the same experiment (join empty chan, set topic "Just like the other channel", then set japanese topic) but with different japanese phrases will not always lead to corruption. Quite frankly I'm a bit stumped as to why this might happen.
(settings used were program wide utf8 enabled, programwide sjis/jis conversion disables, channel font set to display and encode as ut8)