Transaction does not fail, but fields are written wrongly - node.js

Question

I want to store an ordered list of image paths in a Firestore document. The images are uploaded to storage with metadata containing the position and a tag linking to a document. The amount of images to expect is known. Uploading of the images is done nearly simultaneously.
Right know, I do this by storing the list of image paths as an array.
In short: an image is uploaded, using a transaction a document is read, the array is altered and saved again. The transaction never fails, but sometimes an array item disappears again after another image is uploaded.
My two questions are: is an array the best structure in a NoSql database for this problem? Why do array item disappear despite of the transaction?
export const addImageToTag = functions.storage.object().onFinalize(async (object) => {
try {
const tag = object.metadata.Tag;
const position = object.metadata.Position;
const tagRef = db.collection(COLLECTION_TAGS).doc(tag)
const tagSnapshot = await db.runTransaction(t => t.get(tagRef));
const images = tagSnapshot.data().images;
images[position] = object.name;
let uploadCount = 0;
for (let i = 0; i < tagSnapshot.data().imageCount; i++) {
if (images[i].length > 0) {
uploadCount++;
} else {
break;
}
}
let state;
if (tagSnapshot.data().imageCount === uploadCount) {
state = STATE_ACTIVE;
} else {
state = STATE_UPLOADING;
}
await tagSnapshot.ref.update({
images: images,
state: state
});
console.log('Image ', position, ' of tag ', tag, 'added')
} catch (error) {
console.error('Call failed: ', error)
}
});

Answer 1

The second question is answered, actually putting the code in the transaction block did the trick. I had read that referencing to the tag document using tagSnapshot.ref was sufficient, which was false.
export const addImageToTag = functions.storage.object().onFinalize(async (object) => {
try {
const tag = object.metadata.Tag;
const position = object.metadata.Position;
const tagRef = db.collection(COLLECTION_TAGS).doc(tag)
await db.runTransaction(async transaction => {
const tagSnapshot = await transaction.get(tagRef)
const images = tagSnapshot.data().images;
images[position] = object.name;
let uploadCount = 0;
for (let i = 0; i < tagSnapshot.data().imageCount; i++) {
if (images[i].length > 0) {
uploadCount++;
} else {
break;
}
}
let state;
if (tagSnapshot.data().imageCount === uploadCount) {
state = STATE_ACTIVE;
} else {
state = STATE_UPLOADING;
}
transaction.update(tagRef, {
images: images,
state: state
});
if (state === STATE_ACTIVE) {
console.log('All images of tag ', tag, ' were succesfully added')
}
});
} catch (error) {
console.error('Call failed: ', error)
}
});