It appears that recently TMI’s JOIN system was upgraded, and while it works great for channels that exist it relays no data to clients when channels are not joined.
IRC RFC states that for channels that do not exist, a ERR_NOSUCHCHANNEL (code 403) should be sent. This used to not be an issue since TMI would previously join any channel you threw at it, but since the new implementation my client has no idea that the channel cannot be joined (I have my client set to repeatedly retry joining channels [because in the past joins would sometimes fail]).
Edit:
I’m also not sure if it’s intentional, but the new JOIN system appears to still allow you to join the channel “#” (empty string).