While testing the "append data" ACL permission, I wantted to try this out with cifs vfs against a Windows 10 machine as server.
"tee -a" is using O_APPEND but it looks like cifs vfs is not translating O_APPEND open calls to SMB's FILE_APPEND_DATA. So files that have "append only" permission but not "write data" permission will not be "append writable" by cifs vfs in cases where they shold.
O_APPEND is not looked at by the Linux kernel yet, see https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/smb/client/file.c#n374
Let's see if I can change this and make it my first Linux kernel contribution.