The setFCV command may take longer to propagate to all nodes and this in turn can cause fassert in mongos caught in BF-13398.
The proposed fix will add a check that all nodes on config server are actually updated st https://github.com/mongodb/mongo/blob/r4.3.0/jstests/multiVersion/genericSetFCVUsage/crash_mongos_against_upgraded_cluster.js#L33