Hi,
What you see is in fact a DC Offset.
To quote our product manager about this subjet :
"By design, our dicrete-pro mic preamps don't have DC filter. What you see is the continuous DC offset <1Hz, way under audio range. This DC signal has no influence on the audio."
Some DAWs have an utility tool that can remove the offset signals like Utility in Ableton Live.
Regards,
Matthieu