Finally fixed in commit 0d5578c12beec105f5ec113cc50deab703270a82:

  ~$ guix environment --pure guix -- ./pre-inst-env guix build \
     --{check,no-grafts} libreoffice
  successfully built /gnu/store/...-libreoffice-6.4.6.2.drv
  successfully built /gnu/store/...-libreoffice-6.4.6.2.drv
  /gnu/store/25q6nl2l2k3nks0rrh2ah5qsxr3mqy3a-libreoffice-6.4.6.2

Closing, though it remains to be seen whether it works across file 
systems.

Kind regards,

T G-R