It only happened once when I reconnected quickly, but likely the TCP connection was not yet fully closed in that attempt. It resumed functioning in the very next attempt. If that's the behaviour you're seeing for extremely fast reconnects, that's somewhat normal, but it shouldn't happen more than once in a row.
This is NOT somewhat normal. mIRC should be checking the port to make sure it is actually available for use before attempting to use it. mIRC should skip over the currently unavailable port and use the next one in the port range, unless all of them are in use. mIRC assuming the port is available just because it is not using it anymore is a bug.
