Mac Mail, IMAP Accounts & Backup

Does anyone know if there have been any changes to Mac Mail or MacOS in the last few years, with regard to how it handles the local caching of IMAP email accounts? I have an older Mac from 2021 that was connected to an IMAP email account. The backup for the User / Library / Mail folder on this computer is over 50 GB. That is more or less consistent with size of the IMAP account online. I also have a newer Mac from 2023 that is connected to this ***same*** 50 GB+ IMAP email account. But the backup for the User / Library / Mail folder is 0kb. It's just a bunch of nested, numbered folders...apparently with no real data inside. Obviously, there have been normal additions and deletions of email on this account over this time, but it is still a fairly large inbox that should be way more than 0kb on a backup. And it is 0kb on ALL backups that have been made since moving to the new device. Did something change with MacOS or mail in the last few years? Is the mail cache saved somewhere differently now, or is there a specific setting that is required to store the IMAP email cache locally so it can be included in a backup? TY!