Among the dozen of other posts about this specific thing, you'd think that the information would come across other users like yourself. This is not a mIRC bug, it's a change to the twitch network. You now need to request a membership cap when you connect to the twitch servers.
Put this in your script file and it'll work if you restart mIRC.
on 1:connect: {
if (twitch isin $server) {
/raw CAP REQ :twitch.tv/membership
/raw CAP REQ :twitch.tv/commands
}
}
