The o365 securlet's exposed content dashboard keeps a list of exposed files and the associated risk type. When a file is deleted or the share has been removed the securlet will receive notification of the change and clean up the exposed content dashboard in CASB.
When a o365 user is deleted the securlet does not get notification of the file deletion and the exposed content dashboard is not updated in CASB.
A rescan will not clean up the exposed content discrepancy.
ISFR-3227 has been created to request a process to clean up the discrepancy in the exposed content. Please open a support request and ask to be added to the feature request.
Addition information on OneDrive file retention and deletion.
https://learn.microsoft.com/en-us/sharepoint/retention-and-deletion#the-onedrive-deletion-process