I've got the option on in the settings, and it works. If I do a !kick trigger to get kicked by the bot, I'll rejoin instantly...but if I do /k <my name> <reason> it will kick me, but it won't rejoin automatically....

Just wondering if it was a bug, or something I'm overlooking...

mIRC won't auto-rejoin if you kick yourself, the idea (I assume) is that if you kick yourself you don't want to be in the channel.

You can use a script to automatically rejoin the channel if you kick yourself:
on me:*:KICK:#:if ( $knick == $me ) { join # }

You can also have it so you only auto-rejoin if the setting is enabled in options:
on me:*:KICK:#:if ( $knick == $me ) && ( $gettok($readini($mircini,options,n0),8,44) ) { join # }