Create simple email with two Bcc entries.
first is a mailing list
second is a single contact email address
save as template

select 'Templates' folder to check details of template saved correctly. All is ok.

click on template to open in 'Write' window and the second Bcc entry has been removed.

If I reverse the order so the single person is first and the mailing list is second, then the template works and when used both Bcc display.

if I have a Bcc with single contact, followed by mailing list, followed by single contact, save as template and then use, the last contact after the mailing list is removed.

if I have a Bcc with single contact, followed by mailing list, followed by two single contacts, save as template and then use, the contact that immediately follows the mailing list is removed.

Conclude, any recipient entered immediately after a mailing list will be removed when you select to use the template.

Correct. Also, doesn't affect just BCC.