We've opted to close
SERVER-33517 as won't fix, which means the test exercising the scenario must be modified so that it no longer fails.
feature_compatibility_version_lagging_secondary.js avoids this problem by halting secondary replication while setFCV is called on the primary. I suspect it's only necessary to stop replication around the setFCV call. If so, perhaps modify feature_compatibility_version_lagging_secondary.js to do that, too, and add comments about why replication must stop!