Why $cond not working with updateOne and $set in MongoDB 5.0.6

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I have this single document in products collection:

"_id" : ObjectId("629868369f69156e9a0592b4"), "product_id" : "2", "stock" : 4.0, "stock_history" : 1.0, "availability" : true

If I do this:

db.getCollection("products").updateOne(
    {'product_id': '2'},
        $set: {
            stock: 2.0,
            availability: true,
            stock_history: {
                $cond: [
                    {$ne: ['$stock', 2.0]}, 
                    '$stock'
Basically I want that stock field gets updated to 2 and stock_history to old value, that is 4.0.
Instead of this, the stock field gets updated and stock_history field becomes this: (entire document)
    "_id" : ObjectId("629868369f69156e9a0592b4"), 
    "product_id" : "2", 
    "stock" : 2.0, 
    "stock_history" : {
        "$cond" : [
                "$ne" : [
                    "$stock", 
            "$stock"
    "availability" : true
I have noticed that if I use .update instead of .updateOne like so:
db.getCollection("products").update(
    {'product_id': '2'},
        $set: {
            stock: 2.0,
            availability: true,
            stock_history: {
                $cond: [
                    {$ne: ['$stock', 2.0]}, 
                    '$stock'
it works correctly.
But I need updateOne, because later I will use bulkWrite that is supporting only updateOne.
As I can see the difference between my queries with updateOne and update, is that update has the $set object between square brackets, which updateOne does not support giving this message:
the update operation document must contain atomic operators
What am I missing?
Ah, I found the problem.
It works with updateOne also if {$set:...} is put between square brackets.
The only problem was that I was using an older shell, for mongodb 4.0.2 , but my server is 5.0.6 and only from mongodb4.2 the update and updateOne supports aggregation pipeline.
That was the cause for the operation document must contain atomic operators.
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