Ref: https://core.trac.wordpress.org/ticket/64083#comment:61
A commit was made to the 6.0 branch of WordPress core last week. The commit only touched GitHubGitHub GitHub is a website that offers online implementation of git repositories that can easily be shared, copied and modified by other developers. Public repositories are free to host, private repositories require a paid subscription. GitHub introduced the concept of the ‘pull request’ where code changes done in branches by contributors can be reviewed and discussed before being merged by the repository owner. https://github.com/ Actions workflow files, but it resulted in the built files on this branch changing. The same change was committed to all branches between 4.7 and 6.9 yet none of the other branches produced a change to the built files.
Did something change on the build server since the last commit in November that would cause the built files to change? As far as we can tell, everything in the Node dependencies, webpack, browserslist, etc is pinned as it should be, and this is the only branch that produced a different build.