Both stages follow different execution paths and can return different results non-deterministically in a sharded collections topology. Investigate whether this discrepancy in results can be fixed on the extensions side (or if minimal server changes can be made that are specific to the extensionLimit stage).

If so, update the MatchTopN extension to use $extensionLimit (and its relevant tests) and the extension_limit.js test to assert on the results instead of just the result set lengths for sharded collections.