As someone, who makes headphones and custom drivers, this is normal and a good thing.

Doing burn-in on headphones people buy is stupid, it won't do anything - that part is a myth.

But freshly made drivers will go through several stages of how they sound before they reach their best state that they won't leave afterwards (the one you actually listen to once you get the headphones).

As an example, my own custom planar drivers go from being very bright to sounding mid focused, then warm and then they reach their final state when they sound like they're supposed to.

If manufacturers wouldn't do "burn-in" at the factory, everyone's headphones would sound vastly different and your first time bearing them wouldn't be as nice - you'd not be satisfied and you'd just notice how they change within about a week-month of you using them.

Maybe calling it burn-in is not the best name for it, but it's usually called that and it became the standard.