A new update is available within the Office 365 admin center:
Content Type New Button Order supported in the SharePoint modern document library experience
We are enabling support for the Content Type New Button Order feature in the modern experience in SharePoint Online document libraries. This change will begin rolling out today, July 15th and is expected to complete by the end of the month. Previously, document libraries that had this feature configured would automatically revert to classic mode. This will no longer happen, and custom content type ordering will be reflected in the new menu in the modern document library UX. This may result in some users seeing the modern document library for the first time. There is nothing you need to do to prepare for this change. Please click Additional Information to learn more.