iCloud not showing one email in a thread of messages from same sender

I've been sending messages back and forth with a contractor on bid for a project. All the messages go through except for one. This one has the final bid with the contract terms that needs to be signed. Other messages had the same document attachments and went through fine. I saw him send it, and saw it sitting in his sent folder just like the other messages. I just need to get an e-signature on the document.

Another email account is available if necessary, but since this is not time critical, I wanted to see if anyone had an answer as to why this happens. It is not the first time. I have tried rebuilding the mailbox and no luck. This also happens whether on my MacBook or my iPhone, device is not part of the issue I don't believe.