Covered query for $replaceRoot involving missing fields returns different results than collection scan

python buildscripts/resmoke.py --storageEngine wiredTiger --suites aggregation repro_match_replace_root.js repro_match_replace_root.js assert.commandWorked(db.my_coll.insert({}));   const res1 = db.my_coll .aggregate([ { $match: { a: { $not: { $gte: 5 } } } }, { $replaceRoot: { newRoot:{ v: '$a' } } }, ]).toArray();   assert.commandWorked(db.my_coll.createIndex({a:1}));   const res2 = db.my_coll .aggregate([ { $match: { a: { $not: { $gte: 5 } } } }, { $replaceRoot: { newRoot:{ v: '$a' } } }, ]).toArray();   assert.eq(res1, res2);