I have not been able to reproduce this issue so far. In my tests, the quits appears in channel, status, both, or hidden depending on the setting in the Options/IRC/Events dialog.
As far as I can see from the source code, there has been no change to this feature since at least mIRC v6.35, eight years ago.
Are you using any scripts or addons that might be causing this issue?
Which IRC server are you connecting to that displays this issue?
If you install a clean copy of mIRC in your Windows Documents folder, using the "portable" option in the installer, and run mIRC from that folder, are you able to reproduce this issue with it?