Is it mIRC or twitch problem, when you join some channel (twitch.tv) with 10k+ users, you cant recieve any nicks at all. But you still recieve channel messages while it is [#channel (twitch,yournick) [1]].
This is a twitch thing, not mIRC.
EDIT: It's an twitch bug, i had try it in other clients and same results as mirc so you have to contact to the twitch to resolve it.
100% reproduce it.
I have no experience with twitch, but this might actually be intentional: with channels that large, the joins/parts/quits would create an incredible amount of useless noise, and for that reason, some IRC servers support an "auditorium mode" (or something similar) which hides channel users from each other.