Migration assistant didn't import all my mail?

I have all my email from way back, in various folders in Apple mail. Some of it was imported into the computer I am migrating from, a 2009 MacPro running El Cap.

I used migration assistant to migrate from a fresh time machine back up to a new M1 MacMini, but it appears I am missing all the older email, looks like anything prior to 2017 was not imported.

If a folder only has mail older than that, the whole folder is missing on the Big Sur Apple mail app.

How do I get the older mail into the mail app without making a mess of things, i. e. possibly reimporting the 2017 and newer mail and having duplicates?

My working theory, and of course not sure it is correct, is that there's a mail archive somewhere on the MacPro that the MacPro mail app knows about, but migration assistant wasn't smart enough to track down and import into the new mail app.

Thoughts appreciated.