Why is Mac Mail constantly downloading old iCloud mail messages?

Mac Mail is constantly downloading tens of thousands of older messages in my iCloud mail. No other mail account has 52,000 messages.

There are no active Google email accounts. There are two active third party accounts, but they have less than 2,000 messages.

M4 Pro MacBook Pro, Mail 16 and Sequoia 15.5.

Why is it doing this?

[Re-Titled by Moderator]

Original Title: Mail constantly downloading