The copy is to a PDSE, and there is a $UPDATE control statement for the output dataset.
FastCopy is being invoked even with //FCOPYOFF DD DUMMY and FASTCOPY=N specified on a $IEBCOPY statement.
For a PDSE, whether IEBCOPY or FastCopy is used depends on whether or not there is CA PDSMAN interest in the output library.
If there is NO CA PDSMAN interest, then IEBCOPY will be used. If there IS PDSMAN interest, FastCopy MUST be used. CA PDSMAN cannot establish what updates IEBCOPY has done to a PDSE, therefore FastCopy is forced so that CA PDSMAN can see the updates.
For a PDS however, CA PDSMAN can establish what has been updated by IEBCOPY, so it does not matter if FastCopy or IEBCOPY does the work. This means that //FCOPYOFF DD DUMMY will be effective in either case.