To prevent unexpected changes from potentially slipping in after auto-merge is enabled on a pull request, auto-merge is now disabled automatically when new changes are pushed by a user without write access to the repository.
Note: users without write access can still update their pull requests to bring in changes from the base branch without having auto-merge disabled, but auto-merge will be disabled if the update results in merge conflicts that have to be resolved. This is to prevent merge-conflicts being deliberately used as a way to introduce code that hasn't been fully reviewed by the people with write access to the project.
Learn more about automatically merging pull requests when all merge requirements have been met.