I too, found this VERY frustrating, and while I had my own workaround to always copy and paste the bccs into a new email that I wouldn't send, but would save, this was only useful if I remembered to do that. Once I pressed send with a group of BCCs, I could not see them again on that sent mail.
However, I think I have finally figured out a way to retrieve BCCs from a sent email. In the email search box, type in the name of one person you know you bcc'd on that email. The email will pop up (possibly multiple iterations of it) labeled as Trash, and from there, you can see all the BCCs again and re-copy them. I tried looking directly in the TRASH folder and these previous iterations of the sent email were not visible. They only popped up when I searched a specific name.