diff --git a/changelog_unreleased/javascript/10695.md b/changelog_unreleased/javascript/10695.md
new file mode 100644
index 000000000..8d64d832a
--- /dev/null
+++ b/changelog_unreleased/javascript/10695.md
@@ -0,0 +1,40 @@
+#### Process conditional groups in `mapDoc` (#10695 by @thorn0)
+
+In particular, this fixes broken substitutions in HTML-in-JS.
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+export default function include_photoswipe(
+	gallery_selector = ".my-gallery"
+): string {
+	return /* HTML */ `
+		<script>
+			window.addEventListener("load", () =>
+				initPhotoSwipeFromDOM("${gallery_selector}")
+			);
+		</script>`;
+}
+
+// Prettier stable
+export default function include_photoswipe(
+  gallery_selector = ".my-gallery"
+): string {
+  return /* HTML */ ` <script>
+    window.addEventListener("load", () =>
+      initPhotoSwipeFromDOM("PRETTIER_HTML_PLACEHOLDER_0_13_IN_JS")
+    );
+  </script>`;
+}
+
+// Prettier main
+export default function include_photoswipe(
+  gallery_selector = ".my-gallery"
+): string {
+  return /* HTML */ ` <script>
+    window.addEventListener("load", () =>
+      initPhotoSwipeFromDOM("${gallery_selector}")
+    );
+  </script>`;
+}
+```
diff --git a/src/document/doc-utils.js b/src/document/doc-utils.js
index 084844cf4..a57e0a339 100644
--- a/src/document/doc-utils.js
+++ b/src/document/doc-utils.js
@@ -71,26 +71,53 @@ function traverseDoc(doc, onEnter, onExit, shouldTraverseConditionalGroups) {
 }
 
 function mapDoc(doc, cb) {
-  if (Array.isArray(doc)) {
-    return cb(doc.map((part) => mapDoc(part, cb)));
+  // Within a doc tree, the same subtrees can be found multiple times.
+  // E.g., often this happens in conditional groups.
+  // As an optimization (those subtrees can be huge) and to maintain the
+  // reference structure of the tree, the mapping results are cached in
+  // a map and reused.
+  const mapped = new Map();
+
+  return rec(doc);
+
+  function rec(doc) {
+    if (mapped.has(doc)) {
+      return mapped.get(doc);
+    }
+    const result = process(doc);
+    mapped.set(doc, result);
+    return result;
   }
 
-  if (doc.type === "concat" || doc.type === "fill") {
-    const parts = doc.parts.map((part) => mapDoc(part, cb));
-    return cb({ ...doc, parts });
-  }
+  function process(doc) {
+    if (Array.isArray(doc)) {
+      return cb(doc.map(rec));
+    }
 
-  if (doc.type === "if-break") {
-    const breakContents = doc.breakContents && mapDoc(doc.breakContents, cb);
-    const flatContents = doc.flatContents && mapDoc(doc.flatContents, cb);
-    return cb({ ...doc, breakContents, flatContents });
-  }
+    if (doc.type === "concat" || doc.type === "fill") {
+      const parts = doc.parts.map(rec);
+      return cb({ ...doc, parts });
+    }
 
-  if (doc.contents) {
-    const contents = mapDoc(doc.contents, cb);
-    return cb({ ...doc, contents });
+    if (doc.type === "if-break") {
+      const breakContents = doc.breakContents && rec(doc.breakContents);
+      const flatContents = doc.flatContents && rec(doc.flatContents);
+      return cb({ ...doc, breakContents, flatContents });
+    }
+
+    if (doc.type === "group" && doc.expandedStates) {
+      const expandedStates = doc.expandedStates.map(rec);
+      const contents = expandedStates[0];
+      return cb({ ...doc, contents, expandedStates });
+    }
+
+    if (doc.contents) {
+      const contents = rec(doc.contents);
+      return cb({ ...doc, contents });
+    }
+
+    return cb(doc);
   }
-  return cb(doc);
 }
 
 function findInDoc(doc, fn, defaultValue) {
diff --git a/tests/js/multiparser-html/__snapshots__/jsfmt.spec.js.snap b/tests/js/multiparser-html/__snapshots__/jsfmt.spec.js.snap
index 9183dad71..ca455f105 100644
--- a/tests/js/multiparser-html/__snapshots__/jsfmt.spec.js.snap
+++ b/tests/js/multiparser-html/__snapshots__/jsfmt.spec.js.snap
@@ -167,6 +167,67 @@ setFoo(
 ================================================================================
 `;
 
+exports[`issue-10691.js - {"htmlWhitespaceSensitivity":"ignore"} format 1`] = `
+====================================options=====================================
+htmlWhitespaceSensitivity: "ignore"
+parsers: ["babel", "flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+export default function include_photoswipe(
+	gallery_selector = ".my-gallery"
+) {
+	return /* HTML */ \`
+		<script>
+			window.addEventListener("load", () =>
+				initPhotoSwipeFromDOM("\${gallery_selector}")
+			);
+		</script>\`;
+}
+
+=====================================output=====================================
+export default function include_photoswipe(gallery_selector = ".my-gallery") {
+  return /* HTML */ \`
+    <script>
+      window.addEventListener("load", () =>
+        initPhotoSwipeFromDOM("\${gallery_selector}")
+      );
+    </script>
+  \`;
+}
+
+================================================================================
+`;
+
+exports[`issue-10691.js format 1`] = `
+====================================options=====================================
+parsers: ["babel", "flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+export default function include_photoswipe(
+	gallery_selector = ".my-gallery"
+) {
+	return /* HTML */ \`
+		<script>
+			window.addEventListener("load", () =>
+				initPhotoSwipeFromDOM("\${gallery_selector}")
+			);
+		</script>\`;
+}
+
+=====================================output=====================================
+export default function include_photoswipe(gallery_selector = ".my-gallery") {
+  return /* HTML */ \` <script>
+    window.addEventListener("load", () =>
+      initPhotoSwipeFromDOM("\${gallery_selector}")
+    );
+  </script>\`;
+}
+
+================================================================================
+`;
+
 exports[`lit-html.js - {"htmlWhitespaceSensitivity":"ignore"} format 1`] = `
 ====================================options=====================================
 htmlWhitespaceSensitivity: "ignore"
diff --git a/tests/js/multiparser-html/issue-10691.js b/tests/js/multiparser-html/issue-10691.js
new file mode 100644
index 000000000..9761dcb69
--- /dev/null
+++ b/tests/js/multiparser-html/issue-10691.js
@@ -0,0 +1,10 @@
+export default function include_photoswipe(
+	gallery_selector = ".my-gallery"
+) {
+	return /* HTML */ `
+		<script>
+			window.addEventListener("load", () =>
+				initPhotoSwipeFromDOM("${gallery_selector}")
+			);
+		</script>`;
+}