Do you have other data sources in this workbook? If so, it may be joined automatically to those data sources. In which case, the auto joins can be removed like so:
From the top menu , go to "Data", select "edit relationships". Then for each of your data sources, select Custom and see if it's full of fields joined that you didn't want to join. Sometimes, Tableau engine assumes wrongly and you end up with unintended issues.
I am starting a new workbook in a Tableau desktop and connecting the data source.