Preview channel builds are not code-signed and not notarized. That is a deliberate interim state, not a defect in your download.
Code signing and Apple notarization are tracked as a launch blocker. Until they land, Windows SmartScreen and macOS Gatekeeper will interrupt on first open. The file is still the one whose SHA-256 is published on this page.
Use the checksum: if the digest matches, the bytes were not swapped in transit. The OS warning is about our missing publisher certificate, not about a failed integrity check.