User registration API functions return IENGINE_E_DBFULL when amount of user records registered in IDKit database is equal to user limit specified in license which is used (IEngine_GetUserLimit or IEngine_GetLicenseInformation). What is correct behavior.
Additionally IDKit contains protection mechanism to ensure that it's not possible to clear database and load it with another set of users (potentially exploitable with IDKit in-memory database). IDKit stores hashes of all registered user records even after their deletion for some period of time and hash count is used in IDKit's user limit check. This feature is used to prevent a misuse of the license for identification in a larger database than the limit of the database size specified in purchased license.
This feature is disabled sice IDKit v7.3.0.