I suspect you are streaming from the server to the client in that case and it's transcoding the file...
On the server, are your files using mounted drive paths, e.g. "Y:\xyz\somefile.m4a" or "\\server\diretory\somefile.m4a" ?
If the latter (a UNC path), then I think your NUC will just attempt to play that file directly using the UNC path vs. looking for a local mapped drive in the formar, which does not exist and will not be auto-mounted by Windows to that path. In that case, the server will just stream it to the client (if it plays it at all?)
not sure - others more knowledgeable would know, but the fact that it plays fine from the NUC directly leads me to believe you're just streaming/transcoding from the server and you lose Atmos that way.