Sorry, I’m confused. What do you have set up to change mode to away?
The SHM alarm states (arm away, arm Home, and disarm) are completely independent of modes ( initially home, away, and night, but you can add more and change them to whatever you want).
Just changing the alarm state won’t automatically change the mode.
Modes can be changed with a switch which triggers a routine, with a routine, or with some smart apps.
Alarm states can be changed manually through the mobile app, with a routine, or unofficially with some smart apps.
So if you want changing the SHM alarm state to trigger a mode change, you have to have something set up to do that. Core could, but only if you set it up to do that.
There has been a recent bug reported where changing the mode would not then trigger the things that were supposed to trigger from that mode change. But that sounds like the opposite of the problem that you’re describing.