Not sure what I was thinking when I added this, but the fstat handler in vfs_snapper doesn't look right:

2168 static int snapper_gmt_fstat(vfs_handle_struct *handle, files_struct *fsp,
2169                              SMB_STRUCT_STAT *sbuf)
2170 {
2171         time_t timestamp;
2172         int ret;
2173 
2174         ret = SMB_VFS_NEXT_FSTAT(handle, fsp, sbuf);
2175         if (ret == -1) {
2176                 return ret;
2177         }
2178         if (!snapper_gmt_strip_snapshot(talloc_tos(), handle,
2179                                         fsp->fsp_name->base_name,
2180                                         &timestamp, NULL)) {
2181                 return 0;
2182         }
2183         return 0;
2184 }

It returns zero regardless of whether fsp refers to a snapshot or not.
I think this is a cut-n-paste error from shadow_copy2, which does some fiddling about with sbuf for snapshots. The entire handler can probably be dropped.