I have a Fenix 6X Sapphire, which I upgraded from a 6 Pro on 14th Dec 2020.
Within Garmin Connect - Web, I cannot see any activities in calendar for Dec and Jan in Monthly view! When clicking on the month, all I see is a spinning wheel, and nothing loads! They are however visible in weekly/yearly view. Also can see the first 5 days of Dec, in the overlapped Nov view.
Within the Connect Mobile App, I see no activities in calendar for those months, other than move iq events. All events, activities are ticked to view.