Signer URL not working on Mac

We are integrating ESignLive with our ASP MVC site. I don't think this issue is specific to .NET, but I'm not sure where else to post it (sorry if it belongs in a different forum). Our application generates a signer url like this:

signerAuthenticationToken = eslClient.AuthenticationTokenService.CreateSignerAuthenticationToken(new PackageId(packageId), email);
SignatureURL = authClient.BuildRedirectToSigningForSigner(signerAuthenticationToken, new PackageId(packageId));

We then display the signature document in an iframe using the SignatureURL as the src. This has worked fine in Chrome, Firefox, and Edge. However, today I tried in Safari on a Mac, and instead of loading the document, it appears to be redirecting to https://apps.esignlive.com/login (instead of https://apps.esignlive.com/packages/...). This then fails because the login page has the X-Frame-Options header set to SAMEORIGIN, so the iframe remains blank. If I open the SignatureURL in its own window first, the document appears as it should, and then subsequent reloads of our page display the document in the iframe as it should. Do you know why this is happening, or what we can do about it? Thanks.