Thanks, I was able to reproduce this.
This is due to the latest mIRC beta changing from the draft version of
IRCv3 labeled-response to the final version.
In the final version of lable/labeled-response, an ACK is sent from the server to the client every time a client sends a labeled message that does not require a response from the server, such as a /privmsg or /notice. mIRC enables label/labeled-response by default.
I will be adding a check for labeled ACK messages from the server to filter them out in the next beta.
