To keep SharePoint external files (or folders) sharing secure while improving the end-user experience, an update is being deployed (roll out completion scheduled for end of June) to change the ‘one-time password’ functionality for external users which are also hosted on another Office 365 tenant.

With this update, they will no longer need a one-time password but will logon using their current Office 365 credentials when accessing shared files/folders.

This will end with the creation of a guest account on your tenant.