version:6.0.12 4.2.8

db.t1.insert({id:1})
db.t1.insert({id:2})
db.t1.insert({})
db.t1.insert({id:2})

db.t1.find()

{ “_id” : ObjectId(“65a62118842d8bddd7ef9699”), “id” : 1 }
{ “_id” : ObjectId(“65a62119842d8bddd7ef969a”), “id” : 2 }
{ “_id” : ObjectId(“65a6211c842d8bddd7ef969b”) }
{ “_id” : ObjectId(“65a6211f842d8bddd7ef969c”), “id” : 2 }

— There are no nulls in the result set

db.t1.distinct(‘id’)
[ 1, 2 ]

– After creating the index, there are nulls in the result set

db.t1.createIndex(\{id:1})
{
“numIndexesBefore” : 1,
“numIndexesAfter” : 2,
“createdCollectionAutomatically” : false,
“ok” : 1
}

db.t1.distinct(‘id’)
[ null, 1, 2 ]

After adding the index, null values appeared in the result set, causing the application processing logic to change. I would like to ask if this is a bug.