Terra B Welch
Empress of Light
I've never seen it say post plantera, but, to be honest, I haven't looked past the hardmode horde has arisen. I just think you are having nasty luck; because I went for a few days IRL playing for around 5-6 hours each stretch. I'm not saying I spent all of those hours, but it just wouldn't drop. The play through before that it was just dropping like crazy, while the circuits weren't as plentiful, but were a bit more rare.
I've went to the jungle and the corpsethorn and hive backpack enemies spawned, but I am not getting any barbarian spawns.
Where do barbarians spawn? And what are the chances?