It even starts to remove a file before it finish it.

.changeset/forty-dodos-dress.md

@@ -0,0 +1,5 @@
+---
+'astro': major
+---
+
+don't finish the action of the copy before removing all files.

packages/astro/src/core/build/static-build.ts

@@ -343,7 +343,7 @@ async function cleanServerOutput(opts: StaticBuildOptions) {
 	// Clean out directly if the outDir is outside of root
 	if (out.toString() !== opts.settings.config.outDir.toString()) {
 		// Copy assets before cleaning directory if outside root
-		copyFiles(out, opts.settings.config.outDir);
+		await	copyFiles(out, opts.settings.config.outDir);
 		await fs.promises.rm(out, { recursive: true });
 		return;
 	}