It seems like the out of the box Group people web part can't resolve the members of the Members or Owners group in the O365 group that "owns" the site collection. Is there a community web part that covers this need ?
Are you using People web part in SharePoint Online Modern view?
In my test, People web part could resolve the members of the Members or Owners group in the Office 365 group which is connected to the site collection.
Which permission did you use to edit the web part? Please test using a site owner and compare the result.
In addition, check if other Team modern site collections have this issue.