Please let us know the below details.
1. What version of UCM/URM is installed in your environment.
2. Have you enabled DoD as part of Records Management component. Generally if we don't enable DoD then the URM system will allow to check-in the content even though the category id field is empty.
You can check the Catefory or FolderID value for a content in DocMeta table(xCategoryID).
For completeness sake, there is a configuration flag documented that turns this behavior off. Note that using the flag breaks the DoD compliance rule.