As far as I can tell, there have been no changes in recent versions of mIRC that would cause this issue. This issue is reported by some users after almost every new release of mIRC. It has been
discussed at length. It does not appear to be an mIRC issue, since some users with a clean installation will experience it all the time, from some computers and/or some connections, while other users with a clean installation will not experience it all.
There are many possible causes: your internet provider monitoring and disconnecting IRC connections if they think they are suspicious, your internet connection not working correctly, the server experiencing a large number of connections (such as in a
DDOS) causing it to drop connections, the IRC server disconnecting clients due to inactivity, the IRC server not recognizing the client version number (such as in hacked clients with fake version numbers) and disconnecting them, an issue with the IRC servers host, an issue with anti-virus software not recognizing the new version and closing connections it thinks are suspicious, and so on.
If this is happening to a large number of users connecting to the same server, this usually indicates a server issue and you would need to contact the administrators to see if they can track down the problem.