And another powering option... mine runs just fine off my Macbook and Mackbook pro USB-C chargers. Def handy for mixing at the desk.
When you say "mixing at the desk", are you using it as an interface? I ask because I'm unable to figure out how you get audio into and out of the MixPre with the USB-C port occupied by the charger.
Perhaps you're using it as a recorder "at the desk"?
It seems to run fine for vocal tracking in via the USB-A port on the MixPre. So, power into MixPre through USB-C port, either from a Mac charger or Anker-type battery etc..... Then data out of USB-A port on MixPre-3 though a USB-A/USB-C cable, with the C end run intone of the MacBook Pro USB-C ports.
I'm using a phantom powered mic (not that it matters but in this case a Sennheiser 8060) run into 1st XLR on MixPre and tracking directly from the MixPre without recording into Adobe Audition with no issues that I can discern.
This very minute I'm using an Anker to do exactly this.