Apple Mail export/import corrupted my email by copying the same one repeatedly

On an old Mac I had a mailbox in Apple Mail's "On My Mac" section. The mailbox had 42,065 emails in it. I wanted to transfer this to a new Mac. I exported the mailbox on the old computer, transferred the resulting .mbox folder to my new computer, and then imported it in Apple Mail on the new computer. I glanced at the number of emails and saw that it was 42,065. I thus assumed everything was fine and deleted the mailbox on the old Mac and also deleted the exported .mbox folder.

However, I then noticed that all 42,065 emails in the mailbox on my new computer were identical. Either the export or the import corrupted all of my email! Unfortunately, I had no Time Machine backup on my old computer, and the .mbox folder is not in my Trash since I removed it using rm in the Terminal app. As of now, I believe I have thus permanently lost a lot of my emails! Any suggestions?

(Note this question does not regard macOS Monterey, but I was forced to pick a topic to post this question. I couldn't find Apple Mail in the list.)