Currently offloading can not make use of --fallback, if sources are goneor otherwise marked as not available, the machine builds are offloadedto does not fetch the sources.
Recent example is libxt: Running "guix build libxt" gave 410 gone. Itried "guix build --fallback libxt", same behavior. Running "guix build--fallback --no-build-hook libxt" succeeded.Therefore I conclude that offloading has no possibility to deal with --fallback.--