Yesterday, I posted a question on AskUbuntu, and was surprised that I was unable to upvote the answer to it, as the SE account association bonus hadn't been applied.
I have ~1.7k reputation on StackOverflow, and according to https://stackoverflow.com/reputation I achieved the 200 reputation threshold on 2012-04-16. Other accounts created both before and after the AU account have the association bonus applied, but the AU profile does not.
Also surprising to me is that the AU profile never appeared in the network tab of my profile until I posted the question on AU (in fact, I was entirely unaware that I had ever created an AU account, much less one 2 years ago!)
Am I misunderstanding how the association bonus works, or is something else going on? I did find this question outlining a similar issue, though in my case I achieved the threshold reputation before that question was posted 3 years ago.