Interior door noticeably warped after installation

Of course, a new door leaf doesn't cost the world. 120 euros

A door leaf from a climate door only 120 euros, what kind is that? Honeycomb, or tubular chipboard? I don't think it's solid chipboard.
And shouldn't a climate door have a soundproof seal, where is it?

Was the frame measured?

Were both rooms always at the same temperature?

Strictly speaking, a material defect on the part of the manufacturer cannot be present. It is not a solid wood door but a composite. They only warp due to external causes. Typically, these are storage errors. It is enough if this door has been leaned against a wall in a room with slightly higher humidity for a longer period. If doors are not stored lying down or perfectly upright, it leads to such warping. Or differences in humidity between the two rooms. Which basement has exactly the same climate? I, as the manufacturer, would also be stubborn if a material defect is not proven. And 120 euros is the lowest price segment; I would save myself the lawyer if I were not 100% sure what the cause is.

especially since the other 13 interior doors are not warped like that.

But partly also a bit warped? Door to the bathroom? Show the complete picture of the basement door and bathroom door.

Sorry for the late reply. I will take the photos tomorrow and send them to you. But as I said, if at all, they are only isolated and very slightly warped.

And regarding storage, it should be mentioned that the affected door (from the living room to the basement) was installed by the door company and has not been altered since. It was not leaned against the wall or anything like that. The moisture issue should be much more pronounced with the bathroom/hallway door. But it is not.