You assume that is the only problem in the deploy. That assumption seems to have been correct. However, what if another bug had been found that requires a rollb

You assume that is the only problem in the deploy. That assumption seems to have been correct. However, what if another bug had been found that requires a rollback from B to A? Now your rollback will result in a bunch of dead services until they are all on version A. The best option when a defect in B was detected would have been to rollback so that you aren’t forced into a scenario where every fix has to be a fix forward.