Tested on epiknet (roubaix.fr.epiknet.org) and swiftirc (irc.swiftirc.net), which are running version UnrealIRCd-184.108.40.206 and UnrealIRCd-220.127.116.11
I got a spam message in query, when I noticed it later, the spammer was gone, disconnected from the server (which I didn't know) and when I tried to insult him, as I was typing my letters in the editbox, I got the error 'no such nick/channel' displayed in the query window.
I did not investigate further, I believe it's an issue with mIRC and IRCv3 cap stuff, when you type in a query window and the user is not connected you get that error for keypressing (no enter)
Tested on the above network and mIRC 7.75 beta 1109. The connection to the server was made with the default mIRC behavior, I'm not changing anything there.
Makes me think that mIRC implemented ircv3 caps without making them optional, so as both irc networks and mIRC implements them, one's irc experience is changing, if most ircv3 cap features are great imo, the one behind this report is not so great even when working as intended and I wouldn't/don't want to use it (read: i don't want mirc to automatically use it). It would be great if mIRC had a way to specify which cap to use/allow when connecting, both from the server list option and /server. Currently one has to alter the connecting protocol and it's not really a trivial thing to do.