The tag "office-onedrive-client-itpro" focus more on general issues of OneDrive client, but you issue is more realted to OneDrive Admin Center, which is not supported on Q&A forum currently.
To better help you, I would suggest you post a new question on the following forum.
Microsoft Tech Community for Admin Center
The following content is my personal opinion.
Did you enable SharePoint and OneDrive integration with Azure AD B2B?
According to this article, "Secure external sharing recipient experience", if you enable this feature, after the external user enters the one-time passcode, they will authenticate with their account and have a guest account created in the host's organization.
Besides, you set "New and existing external users" for OneDrive, the new external users would have their guest account after passing the identity verification.
I suggest you change the organization-level external sharing setting to "Existing guests", then it would allow sharing only with guests who are already in your directory.
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