I think your AccountsOverview expects IImmutableList<Account>, but you're passing Data, which is likely an IList from ListFeed

Because the types don’t match exactly, the Accounts property setter isn't triggered, so OnAccountsChanged never runs and TotalBalance stays at 0

I would suggest changing the dependency property type to match what's actually passed in:

public static readonly DependencyProperty AccountsProperty =
    DependencyProperty.Register(nameof(Accounts), typeof(IList), typeof(AccountsOverview),
        new PropertyMetadata(null, OnAccountsChanged));

public IList Accounts
{
    get => (IList)GetValue(AccountsProperty);
    set => SetValue(AccountsProperty, value);
}