Apply substitutions to BCC of Bulk Email
When doing Bulk Email with recipient variable substitutions, those substitutions are not applied to what is sent to the BCC address(es), so instead of getting a copy of what was actually sent to each of the normal recipients, the BCC is just the template.
I can imagine a variety of reasons why this might happen, but cannot come up with a reason why anyone would want it to work this way.
Tim Gaunt commented
I agree, it would be good if when adding either CC or BCC using the Bulk Send API call that the CC/BCC recipients get a carbon copy of the To. If sending 1000 emails, I would expect the CC and BCC recipients to get 1000 emails