Some operations, like rebase/squash/update, require Mergify to impersonate a real user due to GitHub API limitations.
In such a case, Mergify impersonates the user set bot_account or update_bot_account to do it, but if the configuration option is not set, Mergify was using a random organization user.
This behavior is deprecated, Mergify now uses the author of the pull request instead.
If the author never logged into the Dashboard, Mergify cannot impersonate it and will not be able to queue, rebase or squash the pull request.
For the queue action, public repositories accepting pull requests from users outside their organization must set a update_bot_account or update_method: merge.
For the queue action, pull requests opened by GitHub App will not be rebased by default anymore, a draft pull request created to check its mergeability instead. update_bot_account must be set to get them rebased.