Why would an email appear and then disappear - just that fast - in my inbox?

This is how to fix problems in Yahoo Mail: https://help.yahoo.com/kb/sln3223.html

Know your email Client. For example, Microsoft Outlook, it might be that you have some rules that all emails will be move upon arrival in your inbox. I am doing it. For example, all emails from someone will fall automatically to delete folder.

You hit delete or it was moved to your spam folder