A likely cause is damage to the camera’s ambient/light sensor used to help auto white balance. Some cameras use a separate external sensor to measure the light falling on the camera and feed that data into AWB. If that sensor is damaged or misreading after the drop, the camera can apply the wrong color correction, producing a pink/red cast.

That fits your symptoms: auto white balance is wrong, but manual/custom or preset white balance is fine because those modes bypass or override that automatic measurement.

So the issue is probably not the lens optics or the image sensor itself, but a damaged metering/white-balance-related sensor or associated circuitry. Since the camera is already going in for service, mention that AWB is consistently shifted warm/red after the drop so they can specifically check that subsystem.