There seems as though there are multiple bugs.
(1) client did not receive onboarding email immediately.
I reported issue after waiting 45 minutes, was told the onboarding email should be received within an hour (even though all resources say it should be received immediately.) Client proceeded to wait several hours, but never received it. Therefore, I had to follow up on the issue the following day with support.
(2) client still did not receive the onboarding email once they "corrected" the issue and sent it. (fulfillment timeline shows it was finally sent the following day, but client still did not receive it. Client cleared caches, browsing history, etc. Checked spam and regular inbox, but no email.
(3) the dev team saying the contact is not there, when it is! Screenshot sent to support.
Please address the root of the problem for all 3 issues to ensure it does not happen in the future. And, what I could do if it does happen again. (potentially I/the marketing agency could get CC'd on every onboarding email so if this happens I can forward it to the client instead of jumping through hoops for multiple hours and for multiple days.)
Also, please let me know if there is something I should've done differently.