Handling distinct_id and user_id Settings for Accurate User Tracking

Hi , Why is the distinct_id showing as the device ID when we already have the user_id for that user? Using the device ID may increase the MTU (Monthly Tracked Users) count, especially when a user changes their device or logs in from another device. How can we ensure preference is given to the user_id, and only update the distinct_id to the device ID if the user_id is not present?