Apple Mail, local messages are blank and rebuild doesn't help

We have a problem on a Mac running Catalina and are using the Mail app for an Exchange account - Local messages don't display in the app.

The initial problem we had with Mail is it would get stuck in 'moving messages', and message sync with the Exchange account was not working. So we tried doing a rebuild, which did not resolve the problem. Eventually removing and re adding the account fixed that.

But, also there are a lot of local messages under On My Mac and for these, the messages appear in the message list. But when you click any messages to view, nothing appears in the message body. Also, search does not find for any messages in that section.

I saw another post that suggested moving three 'Envelope Index' files out of the Library/Mail/V7/Maildata folder and let Mail rebuild. That went through, but, did not solve the problem. Now we can find and open the folders that the actual email messages are stored in on the filesystem, double click them, and the full message will open and is viewable. So it appears the messages themselves are intact. But we can't get the Mail app to fully index them.

We tried selecting individual folders under On My Mac, doing Rebuild, but it doesn't seem to do anything.

Anyone have a clue about this?

Thanks