I'm assuming not a lot are seeing this issue as the people who set up the teams are administrators (or your consulting Workfront partners) and so you would not really see the effects. I've added the user back as a team member. And most of the functionality is back. Some she says, she can't do but she'll do a reboot later just in case things are cached.
As I looked at this issue more, I've ran a report to see the teams and added owner name, entered by and entry date columns. Interestingly, the owner name and the entered by names do not match all the time. In fact, most of the ones I've set up years ago, I'm set up as the owner and the entered by name is empty. The entry date is 12/13/69(?!). It would have made more sense that the owner is empty and entered by has my name.
While adding the user back to the team as a team member may be the resolution at this time, it doesn't answer why the change seemed to have happened overnight and what else does it affect - that's what worries me more.