The behaviour you are seeing is not caused by a bug but in fact is part of the Overlay functionality.
When an Overlay Layout is rendered, it will do so once and not update. This is part of the design and is to avoid a situation where an Overlay set to show a specific Media Item (like a brand logo for example) over the top of the Scheduled Events, briefly flickers while the Media is reloaded repeatedly.
If you would like the Calendar to refresh when scheduled as an Overlay, I would recommend opening your Overlay Layout in the Designer, Edit the Region containing the Calendar and ticking the
Loop checkbox. Save to confirm the change.
If this does not provide the performance you are looking for, you could add a second Media Item to the Region containing the Calendar. This way, when the second item has finished displaying, the Calendar will be reloaded once it is due to show again.