The setFCV command is subject to a series of error which allow to retry the command safely. The router and the drivers can retry these errors, so that they are transparent to the users.

However, in two places inside the FCV handling code these errors are caught and throw again as custom errors, thus impeding such a behavior.

In master and 9.0  SERVER-124469 incidentally fixes the same issue. The issue has been deemed not worth to backport, due to the rarity of the cause mentioned (killOp); however, this could happen during a step-down as well, so it's a much wider issue.