Just updated from 2.11.1 to 3.1.1 and notice this problem. I have three category groups assigned to an article, group 1 (Region) with 3 items, group 2 (Topics) with about 20 items, and group 3 (Edition) with only one item. All three of these groups are assigned to the channel, such as News. When I publish an article and click (enable) the only item for group 3, it never gets saved to the DB. When you retrieve the article it is not checked. I looked at the POST request and noticed that the cat_group_id was being incorrectly set to 1 when it should be 3. And the cat_group_id_3 still had the category posted, but not in array form, rather like an individual (vs array) input. The other two category groups seem to work properly, as I can properly update those with the combinations that I tried.
This is repeatable for any article update I have tried.
To view the front-end, access this URL first. https://dev.lifesitenews.com/server/get-server-permission
The backend can always be reached.