I found a bug with Avast Free 19.3.2369, shields only install, Win 8.1. Previous 19.1.2360 uninstalled & full offline clean install 19.3.2369.
When typing chkdsk c: /f /x in elevated cmd (or powershell), on reboot autochk doesn't run. Confirmed via event viewer boot logs, no wininit executed on boot. Reboot shows press key to abort scanning, then gets to scanning but proceeds directly to windows boot. Never shows "scanning and repairing drive (c): xx% complete." Reverted to 19.1.2360 executes correctly again. Haven't tried 19.2.2364. Initially I though it was probably the March kb4489881 patch...