How: Couple of days ago I hid one category from the anonymous users, by limiting access to trust_level_0 and higher. It was just an experiment and today I switched back to defaults.
What: Every user received a notification for every badge they had ever earned in that specific category. Personally I got about 90.
Sounds like the badge backfill job - it runs daily, I think. Not sure what the best solution here is…this is kind of expected if a bunch of topics become public and thus can have badges awarded.
Note that those badges were already granted. Now they were granted (or notified) for the second time.
Indeed, but the badges were revoked when you hid the category from anonymous users. No notification is sent for badge revocation. When you “unhid” the category, all the badges were granted again.