Hi Curt,
Based on your description, I’d like to confirm whether you are refereeing the Calendar App added by going Site content> Add an app. If so, please try the steps below to check the outcome:
1.Login to the issued calendar as a Site Administrator.
2.Go to Calendar tab> List settings> General Settings> List name, description and navigation> Group Calendar Options.
3.Check whether it has been selected as Yes. If so, select No and click Save.
If it is not your scenario, please help confirm the following information:
1.Please try to add a new calendar app to check whether the issue persists.
2.Please go to Calendar tab> List settings> Permissions and Management> Permission for this list> Check Permissions. Then, check the permission of one end user and share a screenshot similar as below with us:
3.Regarding “I'm even an Owner”, are you the Site Administration of the team site or just the owner of the calendar app.
Regards,
Fraser