It just fetches the file latest-v2.manifest from the channel folder. For canary, that means:
https://updates.duplicati.com/canary/latest-v2.manifest
The file is a JSON file with a signature.
That is likely the problem. The value has persisted, but is not changed after an update check. The versions are likely compared with == instead of >, so it just notices that it is different not that it is older. I have created an issue for this: