Window defective - How can something like this happen?

Good day,
How is this possible? North side and no force applied, the window is 8 years old.
Of course, we have no idea, yes, there is a heat lamp for the changing table nearby, could it come from that? Actually, it should then be 20 cm lower and not at this height, but okay..
Is this only a cosmetic defect? (It is in the children's room)

Let me guess, the window is wrapped in anthracite on the outside ….

Let me guess, the window is wrapped in anthracite on the outside ….

No, it is white, all original.

Let me guess, the window is anthracite tinted on the outside ....

Sorry, we have it like that... but after 9 years no problems at all! Seems to be a myth that still persists ;) You can see a damage from the inside. Exactly at that height! Can you say something about it?

It's just a visual defect, right? If it's not a thermal bridge or leaking, I would leave it.

How old is the window? If warranty, call the window installer. Otherwise, could it be that some kind of lens effect formed inside? That the frame there melted?