Since we update the FCV independently from the server version, there is a period of time when "last-stable" and "last" server versions have the same FCV version. We have a number of tests that expect their FCV versions to be different; these tests should be modified to check the FCV version on "last-stable" and downgrade to an older FCV if needed. The FCVs are defined in jstests/libs/feature_compatibility_version.js as of the writing of this ticket.