Craft Commerce 3 Decrease line Item qty by one?
Try just POSTing the new absolute quantity you want to change to - e.g. if you have 5 in your cart and want to decrement by one, you'd POST something along these lines:
{
    &quot;redirect&quot;: &quot;6bb2a73aac3479c68e0946e4455624e2c1b840bc2b6356074a51ba01ee1234d/cart&quot;,
    &quot;action&quot;: &quot;commerce/cart/update-cart&quot;,
    &quot;lineItems[82503][qty]&quot;: &quot;4&quot;,
    &quot;lineItems[82503][options][note]&quot;: &quot;&quot;
}

That's what we do, and it works.
Result:
{
    &quot;lineItems&quot;: [
        {
            &quot;id&quot;: 82503,
            &quot;weight&quot;: 0,
            &quot;length&quot;: 0,
            &quot;height&quot;: 0,
            &quot;width&quot;: 0,
            &quot;qty&quot;: 4,
            &quot;note&quot;: &quot;&quot;,
            &quot;privateNote&quot;: &quot;&quot;,
            &quot;purchasableId&quot;: &quot;5115&quot;,
            &quot;orderId&quot;: 145817,
            &quot;lineItemStatusId&quot;: null,
            &quot;taxCategoryId&quot;: 1,
            &quot;shippingCategoryId&quot;: 1,
            &quot;dateCreated&quot;: &quot;2020-10-29T11:55:02+11:00&quot;,
            &quot;dateUpdated&quot;: &quot;2020-11-25T11:34:47+11:00&quot;,
            ...etc
}