@deek87 found out that the update_core API of concretecms.com, which every Concrete site are using to check whether the Concrete has upgrade or not, is returning old concrete5.org domain
URL
https://marketplace.concretecms.com/tools/update_core?&APP_VERSION=8.5.5
Then, the current result is.
`````{“version”:“8.5.5”,“notes”:"",“notes_url”:“https://documentation.concrete5.org/developers/introduction/version-history/855-release-notes",“identifier”:“8.5.5”,“date”:“2021-03-17”,“direct_download_url”:"https://www.concrete5.org/download_file/-/view/115591/”}`