Now I checked again, the part not showing is slightly different from the first time.
Yes, some of them are showing but not some are not. some months are perfectly fine, showing every thumbnail pictures. But some months they don't, and only happen either in the first row or the last row of the Calander. I started using Diarum in 2022, so it was only in 2022, but the second time I check, my 2023 Jan and Feb are having the issue.
It seems like it's not a consistent issue, because sometimes it shows the thumbnail pictures and sometimes it's not.
I set Monday as the first day of the week. I am only using it on my MacBook, no other device.