Okay, this is the same issue that we discussed before: every IRC network is using a different method/wording/format, that can change over time, for letting the client know that it has logged on with a registered nickname. This means that there is no way for a client to determine when this has actually happened, and this affects all subsequent actions, such as joining channels, that depend on the user being logged on to the network.
In the current implementation, mIRC waits at most 60 seconds for a notice from "nickserv" before continuing the logon process. mIRC assumes that the first "nickserv" notice it sees is a notification that the nickname logon process has completed. It doesn't check the language/wording/format of the notice because, as mentioned above, every network uses a different one, and it changes over time.
However, because some networks send the "This nickname is registered" notice, even if a client has already sent the nickserv logon (mIRC sends it right after numeric 001), mIRC ignores the first "nickserv" notice from networks if it contains "/msg" or "/nickserv", since that is assumed to be reminder about needing to log on.
For your current issue, you are also seeing a "You have 10 seconds to identify" notice in addition to the first notice. mIRC would have to ignore this notice but there is no simple way to do this without checking for that exact wording, which could change eg. "30 seconds" and mIRC would need to store and update this for every IRC network that uses a different phrase, different language, and so on.
This type of event really needs to be a numeric.
That said, how are you reproducing this? I have tried connecting to Libera.Chat many times with nickserv and have not seen this second notice.
As for your suggested solution: if I set my favorites dialog to join ten channels on connect, your suggestion would require mIRC to send, in a worst case scenario, 120 channel joins in the space of one minute, receiving 120 numeric 477 replies, etc. Even if you were only joining one channel, I would really not want to implement something like this because the client would essentially be ignoring a server error/warning and repeatedly trying to circumvent it.
Have you tried connecting with SASL? That resolves the issues with nickserv logons and autojoins as far as I can tell.
