Not too sure about the 'bug' more than the implementation aspect, but there is a funky thing that goes on when attempting to input japanese in a channel that has mode +c, with jis/sjis conversion checked.
ie: if a channel is set to +c, and you have jis/sjis conv checked, and you type japanese, you get a
You cannot use colors on this channel. Not sent: <msg>
but, if if you uncheck the jis/sjis conv box, and attempt to send the message again, it works.
From this, it would seem that JIS encoding is merely incompatible, and that JIS/SJIS conversion should convert any JIS text to SJIS for transmittal instead of vice-versa (and as a bonus, it would mean, that any text using jis encoding would be legible to people using windows with japanese font/regionality support, as it would be converted to SJIS, a native windows encoding.)
My twelve cents (adjusted for inflation).