Using server-side copy on a SMB share

According to the SMB docs server-side copy should work on recent versions of SMB via cp --reflink When trying this with a server running smbd 4.15 (via crazy-max/docker-samba ) and a share mounted with mount -t cifs -o uid=1000,gid=1000,vers=3.1.1 //host/share/ /share, I always get Operation not supported for cp --reflink /share/test /share/test2. The versions on the client machine are:

~$ mount.cifs --version
mount.cifs version: 6.9
~$ uname -r
5.4.0-125-generic

Why is it not working? Any ideas? Thanks!