I believe you but this is a weird problem. There is no explanation for what you’re describing so we will have to methodically check everything until we find it.
Can you post a photo of the received email (just the top) showing the From and To lines? You can strike out enough detail so we don’t get the addresses but I would like to see that your own email address isn’t on the To line or the Cc or Bcc lines.
Another setting to check is Settings > Mail and make sure “Always Bcc Myself” is not turned on. Can you show that?
The other thing that I’d like to see is a screenshot of your Settings > Passwords & Accounts page. That page doesn’t display personal information so just post bit as is.
If everything is normal in all of those places, you’ll have to start looking through Yahoo’s email settings.