It scorched
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Why the end of the cook is the danger
As water leaves, the syrup gets hotter and more viscous and moves less, so the material against the pan base is no longer being carried away. That is the same acceleration the syrups and sauces properties describe, and here the margin is measured in seconds rather than minutes.
A heavy pan buys you those seconds
Even heat distribution and thermal mass mean the base is closer to the temperature of the syrup rather than far above it. It is the single most effective equipment purchase in this craft and it is not expensive.
Caramelization and scorching are not the same thing
Deliberate caramelization is a flavor you are building; scorching is localized burning that tastes acrid and bitter rather than deep. Knowing the difference by taste takes practice, and the honest test is whether you meant it.
Milk solids burn before sugar does
Which is why a caramel with cream or butter in it scorches more readily than a plain sugar cook at the same temperature. Those recipes need lower heat and more stirring, and treating them like a plain syrup is a common mistake.
Do not step away
Most scorched batches happen during the short absence that was going to be fine. Having everything you need on the bench before the cook starts is the practical version of this advice, and it is the same discipline the production day pages elsewhere describe.
It is a total loss
Unlike a grained batch, there is no reworking it. That makes the cost of a moment’s inattention the whole batch, which for an expensive chocolate or nut product is a real number.