The full mix app does this as well. The only solution I’ve found is to bring the mix app on screen every few minutes. Also annoying, but takes much less time than reloading and re-syncing the app! I’m guessing this has to do with the way iOS handles backgrounded apps. I don’t know if there is anything they can program into MixPad/OneMix to let Apple keep their network connection open, etc, even in backgrounded state. This seems to be a “feature” of the way iOS handles multiple programs running simultaneously. BTW, I believe after 10 minutes backgrounded apps are given even further restrictions by iOS, which may be causing the problem.
I know this doesn’t help you out, but is confirmation that the problem exists. I too would love a solution so we can leave it backgrounded as long as we like, then switch to make an adjustment and go back to whatever the primary task was.