Why doesn't my wired headset / speaker mic button work with Zello in the background on Android anymore?
A wired headset button configured as push-to-talk in Zello and/or wired push-to-talk speaker mics no longer work while the Zello app is in the background or the screen is locked.
Google Assistant update released around November 2020 introduced more aggressive behavior, which overrides handling of the headset button and effectively prevents Zello from processing these events while in the background. Google Assistant is a feature included in Google app (package name com.google.android.googlequicksearchbox).
To restore wired push-to-talk button background functionality, disable Google Assistant for a Wired headphones device.