If using an Android device with a wired headset, Google Assistant might interfere with Zello's event processing when the app is running in the background. This is the result of a November 2020 Google Assistant release that overrides Zello's handling of the headset button.
This interruption may cause a speaker mic/wired headset button that's been mapped to Zello's PTT button to lose functionality when your device is locked or Zello is running in the background.
Solution
To restore wired push-to-talk functionality, Consider disabling Google Assistant.