Firestore, can not get all docs in a sub-collection if the docs have subcollection

My firestore has below data

• /users/<id>/cards/<card-id>
• /users/<id>/holdings/<card-id>/<date>/<data>

I can get all card ids with /users/<id>/cards/<card-id>, but I can not get the same info from /users/<id>/holdings/<card-id>

const existing_card_ids = new Set();
try {
  const docRef = db.collection("users").doc(userId);
  const collections = await docRef.listCollections();
  if (collections.length == 0) {
    return existing_card_ids;
  }
  for (const collectionRef of collections) {
    if (collectionRef.id == "cards") { // <-- NOTICE this line
      const docs = await collectionRef.get();
      docs.forEach((doc) => {
        existing_card_ids.add(doc.id);
      });
      break;
    }
  }
} catch (error) {
  console.log(error);
}

return existing_card_ids;

check the highlighted line, If I check the collection id is “holdings”, then the same code will not work.

I searched many different ways in SO and also asked Gemini, Chatgpt. Anyone knows why this is failing?