Thanks for your bug report. I have not been able to reproduce this issue so far.

Can you provide an example that reproduces this issue?

Your example would need to be a step by step method that I can follow to cause a server to send mIRC a line longer than 1024 characters.

Alternatively, you can simulate incoming messages using /parseline, for example:

Code
test {
  var %text = A long line of text
  echo text len: $len(%text)
  parseline -itqp :nick!user@host PRIVMSG $me : $+ %text
}