When a previous upgrade or downgrade was interrupted, the jstests/hooks/run_validate_collections.js hook runs the setFeatureCompatibilityVersion command is run to finish the FCV upgrade or downgrade. The featureCompatibilityVersion server parameter and document is then read to affirm the FCV upgrade or downgrade completed successfully. mongos doesn't report a featureCompatibilityVersion server parameter and so the checkFCV() function throws in this case.

https://github.com/mongodb/mongo/blob/f0c31b47ed99ef7ff11e79180205b50e439234a8/jstests/hooks/run_validate_collections.js#L69