I seem to have Avast 14.3 now. I disabled automatic updates because I tether from my mobile, whose data is expensive and limited. Auto-updates are still disabled, but it has updated itself anyway, and checks every few minutes. This means my phone runs out of credit without any forewarning and I can't stop it.
14.3 seems to also have removed the silent mode, and is nagging me to delete cache, trash and temp files (like some cheesy 2016 Android 'battery booster' app).
Are these deliberate, permanent decisions Avast? For the moment I have uninstalled Avast so that I can be sure to have credit on my phone, hoping this decision is reversed.