Thanks for your bug report. This is intentional. mIRC used to support userids of any length. Unfortunately, most IRC networks do not allow userids longer than ten characters. If a client tries to use a userid longer than ten characters, it may experience issues connecting to the server. So a limit of ten characters is placed on the userid length. The userid is also stripped of most characters apart from alpha-numeric and a few other characters, as this can cause connection issues as well.
The only way to resolve this is for the maximum userid length to be specified as a token in the pre-logon IRCv3 CAPS Client Capability Negotiation. There is no IRCv3 CAPS token defined for this option at this time, so you would need to suggest it to the
IRCv3 working group.
Update: actually, as noted in another post below, having this as an IRCv3 CAP would not help resolve the issue.
