First, I’m an Avast user and not an Avast Team member and I don’t use a Avast SecureLine VPN.
The underlying problem raised some time ago, is Amazon is protecting its assets, however it does it, it is able to detect the use of VPNs (possibly an algorithm recording numbers of hits from an IP, etc.).
I don’t know if using Split Tunnelling would be harder to detect the use of a VPN or not.
Or try adding the Amazon Prime address to the Avast SecureLine VPN or Avast Exceptions.