How do Office 365 usernames that contain #EXT# get created?

I have a staff person who is collaborating with people who are not on staff. She’s been sharing O365 SharePoint documents and folders successfully for a long time. She successfully shared a particular document with someone but suddenly the person she shared with is getting the following error:
“That didn’t work. We’re sorry, but can’t be found in the directory”

I  would have assumed an account for the external user in the form of: would have been created. but it didn't. I see lots of other user names in that form.
ronfastI.T. DirectorAsked:
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Vasil Michev (MVP)Commented:
Those are created when an invitation is sent and accepted, or when an admin provisions a guest user. Sharing documents doesnt necessarily result in that, depending on the method she used. It's easier to just ask her to re-share the document.

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ronfastI.T. DirectorAuthor Commented:
Not an ideal solution but it looks like it's the only one.

thank you!!
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