Snappy_ndvi export failure

Following on from the unresolved GeoTIFF-BigTIFF export issue, I tried to do a well constrained investigation using the supplied example ‘’ code.
It worked fine, generating readable images using BEAM-DIMAP, GeoTIFF, NetCDF-BEAM and NetCDF-cf ‘getProductWriter’ export format options (code line 40).

However, for ‘HDF5’ product writer option the fie generated had no ndvi or ndvi_flag bands and for ‘GeoTIFF-BigTIFF’ it failed completely the first time it tried to ‘writePixels’

File “/home/spaceworks/.snap/snap-python/snappy/examples/”, line 67, in
ndviBand.writePixels(0, y, width, 1, ndvi)
RuntimeError: java.lang.NullPointerException
Process finished with exit code 1

Can anyone else reproduce this behaviour (updated SNAP 6 on Ubuntu 16.04)?
Is there a reason for some formats failing?
Is there a solution to writing pixels to a GeoTIFF-BigTIFF with snappy?