This bug also affects the hd-media installer.
With hd-media installer, iso-scan will try mount each partition to find isos with the following command:
mount -t auto -o ro $device /hd-media
Since the partition contains preseed file, which very likely contains the iso file, is mounted, this mount will fail, and iso-scan will eventually fail to find the iso file.
The same problem also exists in /usr/lib/fetch-url/floppy, which also mount partitions without umount.
I personally modified fetch-url/file and fetch-url/floppy, added `umount /media || true` before every return path, then the installer works incorrectly.