Thanks for the video, I can see that the vertical scollbar is changing size, however this is actually normal in some situations. It does not indicate that the number of lines has changed. mIRC calculates the size of the vertical scrollbar on-the-fly.
For example, if you resize the status window to the smallest possible size and fill it with several hundred lines and then maximize the status window, when you scroll up, the size of the vertical scrollbar will change.
As mentioned in a previous post, to see the actual number of lines, you would need to use the $line() identifier.
Since there are many possible situations where a buffer can get cleared, eg. a script, parting and rejoining a channel, and so on, I am not sure how easy it would be for me to reproduce this or to even monitor for it. One possible approach would be to use a script that, once a second, counts the number of lines in every window, outputs the count to a debug file, and alerts you if the number of lines has suddenly decreased.
