I'm 99% sure that it behaves correctly.
Here's the test:
IMail original = Fluent.Mail.Text("")
ForwardBuilder forwardBuilder = original.Forward();
MailBuilder builder = forwardBuilder.Forward(
IMail forwarded = builder.Create();
It behaves the same for multiple from addresses in original email.
When creating a forwarded email,
ForwardBuilder does not copy
To collections at all.