We require DLP's LOB_TABLESPACE to be in SecureFile (see Converting your LOB tables from BasicFiles to SecureFiles format) for all DLP versions after 15.5.
One of the features explained was better reclamation of the space.
This creates confusion since the Symantec Reclamation tool was previously used to reclaim space in the database.
Symantec Data Loss Prevention (DLP)
Data Loss Prevention Oracle Standard Edition 2
Oracle's Automatic Segment Space Management (ASSM) is designed for efficiency in the database, meant for reuse and speed.
Oracle space management is not designed to redo the size of the files once it has been allocated.
Deleting incidents allows SecureFile to reuse the space in a more efficient manner, it does not empty the space. Since it does not empty the space, it will not show files shrinking.
SecureFile provides performance improvements to our LOB storage by utilizing Oracle's Automatic Space Management (ASSM).
If SecureFile spent the time required to resize the files, that is time it wastes not able to process requests.
Beginning with Oracle Database 12c, SecureFiles LOB storage became the default in the CREATE TABLE statement. If no storage type is explicitly specified, new LOB columns use SecureFiles LOB storage.
You may also want to review:
Why doesn't the database shrink after incidents are deleted?
Converting your LOB tables from BasicFiles to SecureFiles format in Symantec DLP
Oracle tablespace (LOB_TABLESPACE, USERS, etc.) for DLP is full, almost full, or critically full