Following the screenshot in the documentation https://docs.metabox.io/extensions/meta-box-tabs/
you have to add the tab field in the field group as the first field. If you add another field above the tab field, then it doesn't work.
The issue actually turned out to be that all the items need to be part of a tab. I was thinking that I could have some items outside of all the tabs so I didn't have a tab at the very top. That is what caused the issue.