resmoke's script to reproduce the issue import {configureFailPoint} from "jstests/libs/fail_point_util.js" ; // Uncomment the line below if you like to test the Classic engine behaviour: // db.adminCommand({setParameter: 1, internalQueryFrameworkControl: "forceClassicEngine" }); const collectionName = 'test_bf_34317' ; const coll = db[collectionName]; coll.drop(); const docs = [ { _id: 108, "str" : "foreground deliverables" , }, ]; const options = { collation: { locale: 'ru' , strength: 1, }, }; coll.insertMany(docs); coll.createIndex({ "str" : "hashed" }, options); // $limit stage does not allow non-optimized query use the index, so we can see the different results when the index is used and not used. const pipeline = [ {$limit: 20}, {$lookup: {from: collectionName, localField: "str" , foreignField: "str" , as: "array" }} ]; function run(coll, pipeline, options) { const explain = coll.explain( true ).aggregate(pipeline, options); jsTest.log(tojson(explain)); const result = coll.aggregate(pipeline, options).toArray(); jsTest.log(tojson(result)); } jsTest.log( 'Run with enabled optimization' ); run(coll, pipeline, options); jsTest.log( 'Run with disabled optimization' ); configureFailPoint(db, 'disableMatchExpressionOptimization' ); configureFailPoint(db, 'disablePipelineOptimization' ); run(coll, pipeline, options); The result of the plan that uses a collated hash index: [ { "_id" : 108, "str" : "foreground deliverables" , "array" : [] } ] The result of the query that uses collscan: [ { "_id" : 108, "str" : "foreground deliverables" , "array" : [ { "_id" : 108, "str" : "foreground deliverables" } ] } ]