Yeah so the last 6 times I played it in the tavern brawl, it gave me warlock which isn’t even supposed to be an option at all, so it’s clearly bugged.
Reported a few times on Reddit, and a Blizzard dev acknowledged it there:
https://www.reddit.com/r/hearthstone/comments/c2l6so/in_the_tavern_brawl_renounce_darkness_replaced_my/erljtx8/
The Darkness is too powerful! It will not be renounced!!
But really, we’re aware of this bug and are looking into it.
I suspect it happened because they banned the sets outside the Brawl Block from the random generation pool. The Basic set was among those banned, which internally includes the 9 basic hero powers. That’d leave Renounce with no options to pick from, so it fails and leaves you with your current one.
oooh interesting! Yeah I didn’t try doing it outside of the brawl yet. Thanks!