Just hear me out with this.
I would imagine that any blocks altered when sorting in the TEMP tabalespace would never be (or don't need to) written as REDO. So why can you create a TEMP TBS with the LOGGING option?
I've got a DB that generates REDO all night when theres little or no DML. It could just be REDO generated by the use of the TEMP TBS as it has the LOGGING option set to LOGGING.
(This is 8.1.5 on Windows NT4)