Calendars not connected, when they are

I look after a group of people, that use Calendly for scheduling. It seems recently a bunch are showing as ‘hasn’t connected a calendar’, however their calendars are in fact connected and have been for some years. When I log in to check the user group there are several that show this - has something changed recently on the Calendly side to trigger this? Noting this is not the normal error response, when a calendar has disconnected. How do I fix this, and stop it happening again?