Dissapearing Email

I did a search for disappearing email and saw many instances of this and although I read through the solutions, I don't know if any apply to me so I am asking about it. I received a payment on 7/6 from a client. It was a Zelle payment notification from my bank. Yesterday the same client sent the same payment. When I downloaded the mails from over night, I saw the new payment notification. However I wanted to move the email from the Inbox to a Sales folder just as I had done 20 days ago with the previous payment. I then opened the Sales folder and the mail wasn't there. I did a whole system search by the sender and the subject and all it found was the 7/6 mail. The 7/26 mail is gone, as if it was a hallucination. Since I knew I got the mail, and so I know I got paid, but it bothers me that other mails are being lost as well. I am running Windows 10 and my Tbird shows "up to date". I am running Malwarebytes which allowed the first message through as it did this time. I can't fathom how a mail can disappear just moving it from one folder to another. Help please!