I remember an episode of Dirty Jobs where there was an old candy plant or something that bees had gotten into, and they made blue honey from the syrup that was left over in the plant.
From my limited understanding almost all beekeepers give their bees sugar syrup to help them overwinter anyway so nothing stops them from supplementing their diets in the spring. It’s obviously not ideal since a lot of the other aromatics from pollen will be missing but it’s still a step up from mixing the end product with sugar syrup.
Thicker 'fondant' is for winter. Thinner syrup is for spring before the nectar flow gets going. Typically over winter the colony reduces in size, and the hive is reduced down to a single "brood box".
Supers (extra boxes) are placed on the hive when the colony is producing enough honey to harvest.
In theory the supplemental sugar shouldn't get into the supers.
Rare, but possibly the bees were doing a swindle.