We have this code in the test suite:

def array_filters_enabled?
  $array_filters_enabled ||= scanned_client_server!.features.array_filters_enabled?
end

This looks at the features of the server and, supposedly, figures out if array filters can be used.

On a 3.6 server with 3.4 FCV, this test returns true because features are entirely derived from wire protocol version range. However, the server as it is running does not actually support array filters:

Mongo::Error::BulkWriteError: Mongo::Error::BulkWriteError: The featureCompatibilityVersion must be 3.6 to use arrayFilters. See http://dochub.mongodb.org/core/3.6-feature-compatibility. (72)