Hi ag43,
what do you mean updated windows 8? Is it windows 8.1 or just fully updated windows 8?
By Avast VPN you mean SecureLine Standalone product or add-on in Avast Antivirus?
I tried all websites you mentioned with windows 8.1 (both x86 and x64) and according the results there are no dns leaks at all.
Regars,
HK