Updating Element in the Array using the positional $ operator

Steps to reproduce the problem Steps to show changing field name makes it work as expected. Note the only change is array element field name is changed from "s" to "a" Tried on MongoDB server version: 3.6.2 Steps to reproduce the problem. // code placeholder > db.mycoll.insert({ "_id" : 1 , "arr" :[{ "s" : "ABC" , "c" : "123" , "count" : 1 }, { "s" : "ABC" , "c" : "12345" , "count" : 2 }]})   WriteResult({ "nInserted" : 1 })   > db.mycoll.find().pretty()> db.mycoll.find().pretty() { "_id" : 1 , "arr" : [ { "s" : "ABC" , "c" : "123" , "count" : 1 }, { "s" : "ABC" , "c" : "12345" , "count" : 2 } ] }   > db.mycoll.update({ "_id" : 1 , "arr.s" : "ABC" , "arr.c" : "12345" }, { "$set" : { "arr.$" : { "s" : "ABC" , "c" : "12345" , "count" : 5 }}})   WriteResult({ "nMatched" : 1 , "nUpserted" : 0 , "nModified" : 1 })   > db.mycoll.find().pretty()   { "_id" : 1 , "arr" : [ { "s" : "ABC" , "c" : "12345" , // *** wrong array element updated "count" : 5 // *** wrong array element updated }, { "s" : "ABC" , "c" : "12345" , "count" : 2 } ] }   Steps to show changing field name makes it work as expected. Note the only change is array element field name is changed from "s" to "a" > db.mycoll.insert({"_id":1, "arr":[{"a": "ABC", "c":"123", "count": 1}, {"a": "ABC", "c":"12345", "count": 2}]})   WriteResult({ "nInserted" : 1 })   > db.mycoll.find().pretty() { "_id" : 1, "arr" : [ { "a" : "ABC", "c" : "123", "count" : 1 }, { "a" : "ABC", "c" : "12345", "count" : 2 } ] }   > db.mycoll.update({ "_id" : 1, "arr.a": "ABC", "arr.c":"12345"}, { "$set" : { "arr.$" : { "a" : "ABC", "c" : "12345", "count" : 5 }}})   WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })   > db.mycoll.find().pretty()   { "_id" : 1, "arr" : [ { "a" : "ABC", "c" : "123", "count" : 1 }, { "a" : "ABC", "c" : "12345", // *** correctly updated "count" : 5 // *** correctly updated } ] }