During "Converting your LOB tables from BasicFiles to SecureFiles", you do not see confirmation of converting to SecureFile.
This specific issue has been observed when following "Solution #1" to convert the LOB table to SecureFiles in Oracle.
If there was a message that the conversion operation did indeed process successfully, the most likely cause of the query result is incorrect login (ie. logged in with the "sys" user account when running the query).
The "protect" account should be used for all DLP related queries (unless specifically stated to use "sys").
If you are logged in with the "sys" user account (and the conversion operation did say that it succeeded when you ran it) log in as the Protect user, and rerun the query (select table_name, securefile from user_lobs where table_name like '%LOB%';)