FYI, I managed to go through decompiled Brink Service Tool plus parse some messages my Air Control wall controller is sending and I placed them in a repo - turns out there is another way to set the flow mode, that does not fall back to default value after a minute - it stays on for hours, perhaps forever in my case of Brink Renovent Sky 300 HRU. In case this is interesting for anyone, the message is:
w,,FanMode,FanMode,,,40cb,0101,,,UIR,0=Holiday;1=Reduced;2=Normal;3=High,,