I'm not sure whether .xyz supports triangles as well, but ARR only supports .xyz files for point clouds:
[https://learn.microsoft.com/en-us/azure/remote-rendering/overview/features/point-cloud-rendering
For triangle meshes use .gltf or .fbx.
If your .xyz file does contain point cloud data, and it's still not working, please file a bug report:
[https://github.com/Azure/azure-remote-rendering/issues
If possible, attach a sample .xyz file that we can try out.