If Gmail rejects emails from your domain it is up to you to fix it. Google is not going to change, and enough of your users will be interacting with people on Gmail that you have to fix it. It doesn't help that Google has been pushing people away from running their own email and into Google's services by ever tightening what it accepts over the years. More than one person has given up on their email server because it was a constant battle with Google, Microsoft, and company to not have important emails disappear into the void.

My takeaway is there is no bug. My takeaway is that his test email bounced because he didn't have the reputation Viva does. Emails are handled on a reputation basis, this is why we use email service providers like Sendgrid, Mailgun, Postmark, etc.