Are you gathering statistics on your data? Are you running Transparent Data Encryption, then it’s time to upgrade to Oracle 126.96.36.199.
Pretty cool, eh? So what’s happening here? Statistics that were gathered on the hr.employees table are stored in the SYSAUX tablespace. If you are running Oracle 12.1 or bellow, SYSAUX can not be encrypted; therefore data that should be encrypted (statistics data) is spilling over to the unencrypted tablespace SYSAUX. Oracle 12.2 now supports encrypting SYSTEM, SYSAUX, TEMP and UNDO tablespaces to help prevent the spillage of data.