https://github.com/apple/swift-evolution/blob/main/proposals/0338-clarify-execution-non-actor-async.md によると、actor-isolated-async functionからnon-actor-isolated-async functionを呼び出すと executorを引き継ぐと書いてあるんです（swift5.7未満） (edited)

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - apple/swift-evolution