This is the intended and expected behaviour and applies to older versions of mIRC as well.
This issue may be more apparent in newer versions of mIRC due to a few changes to how sockets are handled, however it is in fact present in all versions of mIRC.
Sockets are asynchronous, which means that they send data in the background when they can. There is no guarantee that data can be sent immediately in Windows sockets. An attempt to send data may in fact block, or send only partial data, in which case mIRC will try to send any remaining data when Windows indicates it is ready for more.
When using sockets, you should never assume that you can close a socket immediately after sending data. You should wait until on SOCKWRITE has triggered, as described in the help file, and check the send queue to see if it is empty before closing the socket.
