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This is similar to https://ask.clojure.org/index.php/11711/set-mvn-local-repo-via-alias, sorry if I should have just added a note there instead of opening another question.
:mvn/repos does not seem to work inside of an alias either -- only at the top level of deps.edn. If this is intentional, it would be a nicer experience if CLI logged some sort of ":mvn/repos is not supported inside of aliases" warning on launch.
I'm going to mark this as a dupe to the other, and you can make your comment there.