The undertones are caused by intermodulation between the frequencies in the two notes. This is normal, part of any amp tone, and increases with any distortion, even just a little bit.
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This is the sentence my brain couldnāt scramble together at 4 AM. 
I guess that could be what is happening here. Although I havenāt noticed it on other amps (models) before.
Luckily, it is easily solved by a low cut filter 
I am curious though if this happens in the actual Diezel amp as well! And I am also curious about the reply from NeuralDSP!
To me this sounds like it is most certainly a differential tone.
When playing e.g., E4 and C4, their respective frequencies: 329 Hz & 261 Hz and when you subtract these two frequencies, you get 68 Hz - which appears to be the exact differential tone when playing those two notes. This is a very old phenomenon
It is not very frequent that people note it , but it can happen on cleaner amplifiers
My Victory Duchess does it also
yeah, thatās a natural reaction of certain intervals. I wrote a song using only intervals that create that 3rd dissonant tone