iCloud for Windows won't upload videos downloaded from iCloud for Windows

A movie shot on an iPhone 13 is synced to Icloud Photos, normal behaviour.

BUT when I copy that shot movie (MOV file) on my Windows (10) with Icloud for Windows laptop to the Icloud Photos folder in my File Explorer, that copied MOV file is NOT synced to Icloud Photos.

According Apple info on the web it should.

Have a bypass. When I drop the copied MOV video file into iCloud Drive for Windows, it is synced correctly. Then from there, I can go into Files on my iPhone or iPad and perform the "Save Video" action, which then syncs into iCloud Photos successfully. This also works for MP4 and MV4 movies.

Question: why is Icloud for Windows not behaving as described?

See Add photos and videos to iCloud from your Windows computer – Apple Support (UK)

Jan Willem