macOS may lose data on APFS-formatted disk images

After files failed to actually make it to a physical disk somewhere, you'd think (hope?) that perhaps, at least, the file would appear smaller on the disk image. This is probably the most alarming part of this bug – because the filesystem structures are stored on a section of pre-allocated space on the underlying disk, the diskimages-helper application has no trouble updating filesystem metadata. So file size, modification date, permissions, etc – all of those attributes are fine. In yet another bizarre twist, we found that many times a truncated file would even validate a checksum test.