This arises as a side-effect of WPCS 3.0 which privileges WP_Filesystem over the native PHP processes.

A filter 'document_use_wp_filesystem is implemented which if it returns true will try to use the WP_Filesystem to output the file to the user. It will fall back to use PHP readfile if the access cannot be used.

This is of limited use as readfile is only used if the compression options have not been used.