Our client has multiple SharePoint account for each of their clients.

1. ClientA sent them a SharePoint link in an email.
2. Our client clicks the link
3. SharePoint opened the document without any login prompt using the SharePoint account of ClientB
4. ClientA informed our client that they got a notification that the file was attempted to be accessed by ClientB

This is a breach of their confidentiality agreement as now ClientA knows that ClientB is their client.

Credential manager had no saved web passwords. I used a chrome extension to wipe the cookies from clienta.sharepoint.com and when we visited the link, it came up with an error page that contained the User as ClientB. Only once we completely cleared chrome of all cookies, did it finally bring up a login box when visiting the link. 

I'm trying to find out if this is some sort of bug in SharePoint, or if our client is using the system wrong?