@Paul I am curious to learn how you have done your archival. Or was it importing maybe?
As I am doing some tests on email import and archive just right now, I can tell you that on my Catalina setup with DT3.5.1 things seem to be working fine, this is the result of the archive of my test Gmail account:
New email tests is a new database I have created specifically for this kind of tests, and by selecting “Archive Mailbox“ this is the resulting structure. Granted, I have no labels/folders in my setup, but as you can see the full structure of the mailbox has been correctly archived.
I had to fight a little to understand the real difference between import and archive but this old message from @BLUEFROG made things clear to me:
Curious to see how this develops.
Bye, Luca