It darkened

Where it comes from

Mostly time at temperature: a long finish, a batch held hot while bottles are filled, or cases packed tight and left to cool slowly. Each of those adds thermal history after the point you thought the product was made.

Why it matters more here

Maple syrup is graded into four color classes by light transmittance, and they are printed on the label. A batch that darkens past the boundary you printed is mislabeled, which is a different kind of problem from a buyer preferring a lighter syrup. The classes also have flavor descriptors attached, so the claim being made is about taste as well as color.

It continues on the shelf

Slowly, and faster warm and in light. A syrup labeled at the edge of a class can cross it during its own shelf life without anybody doing anything wrong, which is an argument for labeling to the middle of a class rather than to its boundary.

What to do about it

Shorten the hot phase, cool deliberately rather than letting cases find their own way down, store out of direct light and somewhere stable. Then grade at bottling rather than from the memory of what the batch looked like in the pan.

Grade from the bottle, not from the pan

Color is judged on the finished, filtered, cooled product, because every step between the pan and the bottle changes it. A producer who assigns a class from how the batch looked while boiling will be wrong in one direction consistently, and consistently wrong is still mislabeled.

If you cannot grade per batch, the practical answer is to print the grade statement separately from the main label, on a sticker applied at bottling. It costs a step and it removes the whole class of problem where a label order outlives the batch it described.

References