I suppose the disting attenuverter is entirely implemented in the software. The issue here is probably in the analog part of the module.
All inputs on the disting are analog meaning the input circuits see only analog signals even if it is a logic/digital signal you feed it with. The setup of the disting defines how the signals should be interpreted once they are digitised and ready to be further processed in the digital domain.
From my experience in electronic circuits design (a long time ago) I know that if you exceed the input specs, there’s all kinds of issues that could happen and could result in a digital representation of the input signal that is totally corrupted and seemingly uncorrelated to the input signal.
This needn’t be the case with the disting, so you could try using the internal attenuverter.
But I think it will prove you'll need the 321.