I don't know if this a new feature of a bug, but either way it's incredibly annoying.
I've only recently got around to updating to 7.72 from 7.66 due to forgetting to update on the rare occasions I restarted mIRC, but I've noticed that somewhere between that gap there was a change which now causes channels that are +i* to close if I'm unable to rejoin them on reconnect.
I can't see anything in the versions.txt
which would account for this.
I did see...
34.Added "Keep channels open on part" option to Options/IRC dialog.
If you part a channel and its window is kept open, it will not
be rejoined on connect.
But even checking that didn't solve the situation.
*and maybe +k, etc. as well, but unsure of those.