On 10/22 I ran a query on the Data Science SEDE. In the Posts table I selected some columns, FavoriteCounts included. Looking at the results from the .csv I see a lot of different unique values for that column. I ran a similar query today and now it's only Null or 0. Here are two matching rows. You can see I left out the ID column the first time I ran this, hence rerunning it today. Earlier today I had wondered if it had something to do with the migration from Bookmarks to Saves, but this post says the migration was complete at the beginning of October.