When compiling with NETCDF=4 the iceberg restart files are successfully written but are actually unreadable. ncdump returns
ncdump: MOM6-examples/ice_ocean_SIS2/SIS2_bergs_cgrid/RESTART/icebergs.res.nc: NetCDF: HDF error
The model is unable to read the iceberg files. @wfcooke first spotted a related/similar problem and has volunteered to provide an XML to reproduce the problem if needed.