Apple Mail search not finding emails that I know are there

For a year or two (that's through several different OS) Mail search is not finding emails which I know are there because I've saved in specific folders because they are important. When I look in the folders they're not visible. Nor are they in the Trash. This is happening so much I know it is not an occasional accidental deletion on my part.

I've tried rebuilding some of the affected mailboxes but that does not help.

This week I had two important emails in my Inbox completely disappear.

I don't know if this is related, but sometimes when I look at a thread in my Inbox all the emails show exactly the same content, usually that of the most recent.

I love the Apple ecosystem and don't want to go through all the hassle of changing to another app, but this is getting to be a real nuisance.

Any suggestions as to how to resolve this and perhaps find lost emails will be appreciated