So the "value" of a commodity is fixed at the time of production.
But what is "value"?
It's not the utility ("use value").
It's not the market price, which changes over time.
Marx and others try to map "value" to price (Transformation problem) and try to measure exploitation of workers using profits based on selling prices, so there's an attempt to consider this "value" as the correct, instrinsic worth of a commodity.
Consider a brand-new in-the-box iPhone. It has a fixed labor-theory-value, yet its price and worth go down every year as new models come out.
How does labor theory of value help to understand this?
Mainstream economists had moved to a subjective value model even before Marx's death. See the "Marginal Revolution".
This explains that the value of the old model iPhone is reduced because it is less useful than competing newer models, reducing the demand for the older model.
Yet according to labor theory of value, its value is determined by its production.
Thus "value" is best understood as a measure of a cost of production, not as a measure of the "worth" of a commodity.
Wasteful labor? More cost, not more worth.
Increased skill? Reduced labor cost to produce.
Better tools/automation? Reduced labor, reduced cost.
Where's the misunderstanding?