The customer uses a wildcard index on a collection with nested documents/sub-document fields.
 
For a sample document, please take a look below.

db.collTest.insertOne({
"identifier" : {
"prids" :
{ "MoveMoneyId" : null }
}})

Wildcard Index definition: 

{ 'identifier.prids.$**': 1 }

As per wildcard index documentation on unsupported queries and aggregation, the inverse (not null ) is definitely not supported, but null should be.
 
So when we check the below query explain() stats, we see COLLSCAN instead of IXSCAN.
 

db.collTest.find({'identifier.prids.MoveMoneyId': null }).explain("executionStats") 

 
The query undergoes the IXSCAN stage for any other value except when using null.

As a workaround, I could mitigate this behaviour by using the BSON Type check for null, and it used the wildcard index perfectly.

db.collTest.find({ "identifier.prids.MoveMoneyId": {$type:10}}).explain("executionStats")

 
I am attaching explain() for both if it needs to be reviewed.