Power BI tenant settings: block classic workspace creation = Enabled.
1- Create an Office 365 group (e.g. "Group Test") and include members. (At this point, no workspace in Power BI). ok
2- Create a NEW workspace, for example: "BI - Test". (At this point, only showing "BI - Test" workspace). ok
3- Go to the "Access" tab and include O365 group "Group Test" as Viewer to the "BI - Test" workspace and this is where the problem occurs. The "Group Test" O365 group appears as a Classic Workspace and now you have 2 workspaces "BI - Test" and "Group Test".
Best regards,