Compilation failure on aarch64 with Arm Compiler for Linux's flang

[dslarm]

Using Spack to set up config and compile, the following error occurs: 5 errors found in build log.

It makes no difference if I disable the non-standard float16 - using an arg to cmake of: -DHDF5_ENABLE_NONSTANDARD_FEATURE_FLOAT16:BOOL=OFF

5 errors found in build log:
     3047    F90-S-0081-Illegal selector - KIND parameter has unknown value for data type  (/tmp/user/spack-stage/s
             pack-stage-hdf5-1.14.5-rzhstpbozkrcypkkfn5ntevpg2gwfof7/spack-build-rzhstpb/fortran/static/H5_gen.F90: 604
             9)
     3048      0 inform,   0 warnings,   1 severes, 0 fatal for h5pget_kind_16
     3049    F90-S-0081-Illegal selector - KIND parameter has unknown value for data type  (/tmp/user/spack-stage/s
             pack-stage-hdf5-1.14.5-rzhstpbozkrcypkkfn5ntevpg2gwfof7/spack-build-rzhstpb/fortran/static/H5_gen.F90: 609
             7)
     3050      0 inform,   0 warnings,   1 severes, 0 fatal for h5pregister_kind_16
     3051    F90-S-0081-Illegal selector - KIND parameter has unknown value for data type  (/tmp/user/spack-stage/s
             pack-stage-hdf5-1.14.5-rzhstpbozkrcypkkfn5ntevpg2gwfof7/spack-build-rzhstpb/fortran/static/H5_gen.F90: 614
             5)
     3052      0 inform,   0 warnings,   1 severes, 0 fatal for h5pinsert_kind_16
  >> 3053    make[2]: *** [fortran/src/CMakeFiles/hdf5_fortran-static.dir/build.make:325: fortran/src/CMakeFiles/hdf5_f
             ortran-static.dir/__/static/H5_gen.F90.o] Error 1
     3054    make[2]: Leaving directory '/tmp/user/spack-stage/spack-stage-hdf5-1.14.5-rzhstpbozkrcypkkfn5ntevpg2gw
             fof7/spack-build-rzhstpb'

This is using Arm Compiler for Linux - which is LLVM derivative but the NVIDIA/PGI fortran Flang.

As the float16 is disabled, it ought not to be trying to generate it?