I would just like to call attention to this bug report, as this issue remains as of 6.31, and at least for me is a show-stopper which significantly impacts my ability to use mIRC on a daily basis.
The basic issue on my end is that when using Japanese WITHOUT UTF8 enabled, as is the usual method on Japanese channels and networks, the character at the intersection of a line split becomes incorrectly decoded and split into two garbage ASCII characters. An example of this behavior can be seen here:
The point of attention here is the small 't' which can be seen on the third line down, which is being displayed instead of the correct Japanese character as seen further down on the UTF8 line. Occasionally this garbage text is more troublesome ASCII characters which can negatively affect all of the characters that follow.
I would be extremely pleased to see this fixed.