We have also discovered, that this exact issue can be solved by different positioning of the phone - if the bottom of the phone (where mic and speaker are) is not touching any surface, the artifacts are gone. So try to put the phone on a book or something similar with the bottom of the phone over the edge. If you are not hearing the sonar itself during tracking, but it only gets recorded, this usually helps.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org/faqs/sonar_recorded.html