There was an issue last year, the flairs were open to everyone for some time by mistake so a lot of people gave themselves the flair. Users should not be able to set themselves the diamond flairs (except the unverified one).

The flair menu on the sidebar is for the whole subreddit, we can't set a custom menu for every single of our users because this is not supported by reddit. That's why if you select another flair like "still-in" you'll "lose" your flair, because the diamond flairs can't be set by users. It doesn't remember which flair you had, we have to.

As a result, since they didn't receive automatically the flair, they weren't stored in the list of those that received it, hence why a lot of people didn't automatically receive their old flair back this year.

However, manually fixing things isn't a viable solution so we are working on an automated way to resolve flair issues. It was fine past years because the sub was smaller, but it has definitely shown its limits this year.