This was changed because the behaviour was not consistent. Using /loadbuf or /filter would initially load that number of lines into a window but the moment a new line was added to the window after that, the buffer limit was applied and lines were trimmed to the buffer size.
The latest public beta of mIRC v7.11 allows you to get around that with the /window -jN command, which can be used to set the buffer limit for a particular window.
