Contents of mail messages missing on brand new MacBook Pro

I just replaced my old MacBook Pro with a new one. Old machine (mid-2010) was running Sierra. New one, bought last week, is running Catalina.

I used Time Machine to move the contents from a backup of the old Mac to the new one. Now I'm finding that I'm missing the content of many many many email messages. I see that people have been told to install a clean version of Catalina to fix the problem, but this is a brand new machine so must have come with a clean version. I also see that people have been told to restore to a previous operating system. I don't know if I can downgrade to an older operating system that was never installed on this machine. Nor do I know if that will fix the problem, even if I can do it.

What is Apple doing, releasing an operating system that screws up a major application that everyone needs, like mail?

Is there a fix for this?

I know I can call Apple support because this is a new machine, but in the past I've found they often leave me in worse shape than I was before I called.

Does anyone know how I can get my mail contents back? This is absolutely critical. I'm going to have to go back to the 2010 machine if I can't reliably use mail.