Personally I don’t use system restore, - preferring to use drive imaging software - so I can’t really give you a definitive answer as to why this happened.

Avast on Installation I believe creates its own Restore point (but I don’t think it would clear old restore points), is that avast restore point there ?

If so you should still have the fall back to as it was prior to avast installation.

The older the system restore point is the less value it has, periodically people create a new restore point and clear old restore points using the Clean Up function.