Busy agents tend to skip steps which they should not. That is between BA and them (or in the case of interline, some other carrier). Nonetheless, the anecdotal evidence that a practice sometimes works is not something I would ever rely on or argue about.

If you are charged, don't argue. If the issue deals with not interlining, make certain to allow time to retrieve and check your bag at the intermediate point as you are not a connecting passenger.