db={"BulkTests":[{_id:"65b82f4b13f7b6653662795d",testDateTime:"2024-01-29T23:05:47.940+00:00",testId:19},{_id:"65bac13912032ee4eae1d47f",testDateTime:"2024-01-31T21:52:57.185+00:00",testId:20}],"Testers":[{_id:"65bc139856f9734f1d19285d",name:"FooBar Bizzle",testResponses:[7,19]},{_id:"65bc139856f9734f1d19285e",name:"Baz Bar",testResponses:[20]}]}

db.Testers.aggregate([{$facet:{BulkTests:[{$lookup:{from:"BulkTests",let:{},pipeline:[],as:"BulkTests"}},{$unwind:"$BulkTests"},{$group:{_id:null,maxValue:{$max:"$BulkTests.testId"}}}],Testers:[{$project:{lastTest:{$arrayElemAt:["$testResponses",{$indexOfArray:["$testResponses",{$max:"$testResponses"}]}]},name:1}}]}},{$unwind:"$Testers"},{$match:{$expr:{$ne:["$Testers.lastTest",{$arrayElemAt:["$BulkTests.maxValue",0]}]}}},{$replaceWith:"$Testers"}])