Sync with Icloud-Photo folder on Windows works, but photos are not transfered to cloud

[heckh00u]

Hello everyone,
I am trying to synchronize OneDrive images with iCloud photos using FFS on Windows 11.
FFS works fine, selected files are copied correctly to the following path:
C:\Users\xxxxx\Pictures\iCloud Photos\Photos
But the photos added by FFS into this directory do not appear in iCloud.
When I manually drag and drop photos into C:\Users\xxxxx\Pictures\iCloud Photos\Photos, these photos are uploaded to the cloud and appear there too.
The Photo folder and the photos are set to "always remain on this device" and I run FFS with admin rights.
Am I doing something wrong?
Is there a difference between drag and drop and the FFS copy method?