Hey Neil
The NFS workaround does indeed work.
The rendering over and over is a bug in FCP-X when using Fill instead of non or FIT in Spatial Conform with some codecs or media external to FCPX library.
the folder left behind with compressor ( internal share in FCP-X) appears to also be a bug.
Local media exported to the NFS share ( so fcp-x isn’t using anything on the NFS mounted volume) still leaves behind the *tmp folder.
I’m putting the disappearing files down to use error… !
Nice 4K demo at NAB btw.. caught the vids online.
S