Sorry if this is out of touch, but I wonder if mIRC will lose some of its cached connection parameters from the previous connection, for certain types of reconnect failure errors. I note similarly that after certain failure to reconnect, that logs no longer go to the network's status log, but to a generic status log file. I estimate that mIRC may forget its connection cache at that point, except for the server address and port.

(the generic status.log thing has been like that forever)

Well. At least I won lunch.
Good philosophy, see good in bad, I like!