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In my AD authenticated, server 9.1 environment:
If I use a REST login using '/api/2.0/auth/signin', then in ...\Tableau Server\data\tabsvc\logs\httpd\access.*.log I get a line like "POST //api/2.0/auth/signin HTTP/1.1" that contains my IP address and a session ID. If you search for that session ID in ...\Tableau Server\data\tabsvc\logs\vizportal\vizportal-*.log, you'll find the username.