Hi,
A process/user has to run Avast uninstaller for it to be removed. It may happen (rarely) that Avast is removed after a major system update (incompatibility issue).
Could you provide us with some logs, please?
To generate the support file, please see this link.
https://support.avast.com/en-ww/article/Submit-support-filePlease paste the support file id here.
If the logs still exist, we should be able to determine what happened.Location: C:\ProgramData\AVAST Software\Persistent Data\Avast\Logs
After you have created the logs, you can Restart your device.
It might be that an Avast program update did not finish.