I use the "Group calendar" web part within SharePoint with "proper" Office 365 groups with no issues.
I've just created a new Team and everything appeared to provision correctly. I have a SPO site, calendar has appeared in Outlook client and I can access the calendar through OWA.
When I add the "Group Calendar" web part it shows the group name but the web part just reports "There aren't any events scheduled." (before anyone asks, yes there are events in the calendar :)).
In this case I created the Team via the desktop app, provisioning this way hides the group from Outlook. I tested creating a team through the Admin console and this time the web part worked as expected, but I do get the group listed in Outlook.
I am guessing there must be a permission thing going on here, is there any way around this for app created Teams?