Wrapping it inside a <ResourceDictionary> did the trick! Now the bindings are working again.
Thank you for the prompt reply!

To avoid spamming XAML files with confusing AnchestorType bindings, we were using DepencendyObjects as Binding Proxies as described in this Blog: https://code.4noobz.net/wpf-mvvm-proxy-binding/ Our implementation: public class BindingProxy : Freezable { #if !NOESIS protected override Freezable Creat...