is this a bug?

Try using the "generatedLinkToPackageViewForSender" url from this as the source for your iframe, instead:

string senderAuthenticationToken = eslClient.AuthenticationTokenService.CreateSenderAuthenticationToken(packageId);
AuthenticationClient authenticationClient = new AuthenticationClient("https://sandbox.esignlive.com");
string generatedLinkToPackageViewForSender = authenticationClient.BuildRedirectToPackageViewForSender(senderAuthenticationToken, packageId);

Hope this helps. Let me know.

This works in a sense that shows package information same as on sandbox site. It was working before like this. I could verify that draft package is created. The problem was to integrate this into our site. I don't want to see e-SignLive logo or navigation buttons. I want to go straight do a document or documents uploaded to create placeholder for signatures and then continue with workflow.

If you want to use the URL that you've specified, you'll need to create a sessionToken:

eslClient.SessionService.CreateSessionToken(packageId, signer);