Update: I joined a Gmail forum in hopes to find an answer to my problem. From what I’ve seen, it definitely seems to be a Mac issue, not Gmail. Others are experiencing the same thing. One Mac user wrote:
Emails disappearing from folders
“When I open a folder in gmail, all my stored there disappear. Have tried all the solutions offered, including two hours “chatting” with someone at Google and then going through all the remedies they subsequently sent. I’ve lost years of emails and there is no explanation.“