From the link - I wouldn’t think sounding “curt” in his responses could be considered “normal” conversation..Perhaps he was always “curt?”
“”According to the Times, there were normal conversations heard on the cockpit audio recorder prior to the captain leaving. Robin said the co-pilot’s responses sounded “curt” during a discussion about the planned landing of the plane in Dusseldorf.””
Maybe his manner changed when the captain addressed landing issues because that was of no interest to him, as he know they wouldn’t be landing