Given that there is such an abrupt cut-off, my guess is that it’s to do with the audio sample rate being lower than the sample rate that Audacity is recording. In digital audio, the frequency range is limited to half the sample rate.
Do you mean that you are recording from CleanFeed with Audacity?
The CleanFeed website says that CleanFeed has “Built-in recording”, so I don’t understand where Audacity fits in.