How can I preserve BCC information from iOS Mail?
This also occurs with other mail clients on Gmail account.

After a bit of googling, it appears the issue has existed for years, and is likely related to Gmail behaviour with SMTP and IMAP. My guess is that Google create the “Sent Messages” entry from what the SMTP server gets in its DATA command, and discard anything a mail client may want to save through IMAP. Doesn’t seem it will be changed anytime soon.

If this is a rare occurrence, you may want to log into gmail.com when you need to send a message with BCC. If this happens more often, you may want to use the Gmail app or another Gmail specific client.