The exact wording of the spell is "any cube", not "each cube"." Any 5-foot Cube of webs exposed to fire burns away in 1 round, dealing 2d4 Fire damage to any creature that starts its turn in the fire."
So each cube deals damage.
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It's the exact wording of the spell!
In other words, any cube of the web that is exposed to fire burns away. There are no cubes of the web that are immune to being burned.
The fire deals 2d4 fire damage to any creature that starts its turn in the fire. Nothing in the text of the spell states or even implies that there are multiple fires, one per cube, each of which is a distinct damaging incident.
An analogue in statutory drafting would be:
*If any part of a car is not fit for purpose, it is unroadworthy, rendering the driver of the vehicle liable for operating an unroadworthy vehicle."
That's probably not the best drafting of all time. But the most natural reading of it is not that the driver commits one offence of operating an unroadworthy vehicle per unfit part of the car; but rather that any part being unfit is sufficient to render the driver liable - ie there are not parts which can be unfit yet the driver not be liable.
It is the burning away that deals the damage, not the cube. This is clear both from syntax, and from common sense. And again, nothing in the syntax entails or even implies that multiple cubes entail multiple events of burning away.The sentence structure is clear. "each cube" is the subject of the verb "burns" and is also the subject of the verb "dealing". Fire is not the subject of the verb dealing . If that was the intention, the sentence is not written properly.
By analogy, in my imaginary statute it is the being not fit for purpose, and hence unroadworthy, that renders the driver liable. But there is no entailment or implication that the state of being unroadworthy is distributive and cumulative.