That sounds like a google account. They remove the BCC recipients from the sent folder. I have no idea why they do it, they just do.

Actually, no. I'm using Thunderbird (current version) to manage emails run off my ISP's server, and it is not Google. But, I'll ask them about that. Thanks.

Try redirecting the sent copies in account settings to "Local folders > sent" That will leave the copies on the local account only and not place them on the server IMAP account at all. If it is like Google, the copies will still appear in sent as they add them as they pass through their servers on the way out. But they will continue to be missing the BCC and the local copies will retain them.