AD7908 full scale value

Hello.

For the conversion output of the AD7908, the value at full scale input fluctuates.
The AD value changes according to the voltage of the previous channel.
Measured at 5 V full scale.
Condition 1 VIN 1 = 5 V, VIN 2 = 5 V AD value 255
Condition 2 VIN 1 = 0 V, VIN 2 = 5 V AD value 252
It is the same result even if the time of VIN1 and VIN2 is sufficient.
If the voltage of the previous channel is 0.5 V or 1 V, the AD value is 255.
The input buffer circuit is also considered, so I think that there is no particular problem.
What can be considered as a cause of this phenomenon?

Best regards