Thanks to Anton's suggestion, the fix was relatively simple — see ASDF 3.1.0.56, commit eb2da723.
I also committed some improvements initiated during the walkthrough.
Anton: can you run cl-test-grid against that? If you have time, also try with (uiop:enable-deferred-warnings-check) and also try with :uiop in the :use list of :asdf-user.
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