User files stored in SharePoint Online and OneDrive could be encrypted by a malicious actor if a vulnerability of versioning system is being used. If an account is compromised, the number of versions can be reduced to 1, then the files would be encrypted and cannot be retrieved without decryption keys.
Having offline backups of the OneDrive and Sharepoint repositories is the safest method to protect data stored online by Office 365 or Microsoft 365 users.