[SalesForce] JSON.deserialize on custom sObject not parsing related fields from lookup relationship

I've got a map whose values are a wrapper class. The wrapper class holds a custom sObject list, and the sObject in turn was queried to include related fields from a lookup relationship.

Problem is, when I use JSON.deserialize(jsonString, Map<Id, MapWrapper>.class), although the custom sObject is parsed, the related fields via the lookup relationship are not.

In prettified JSON, my serialized wrapper class (actually a map of ID to wrapper) looks like this, with the missing related fields highlighted :

{  
   "0011b00000Z6Pa8AAF":{               
      "entity":{                        
         "Id":"0011b00000Z6Pa8AAF",
         "Name":"Mohyee Holdings LLC"
      },
      "periodList":[  
         {  
            "Id":"a141b0000006lntAAA",
            "LLC_BI__Statement_Date__c":"2018-12-31",
            "LLC_BI__Source__c":"Tax Return",
            "LLC_BI__Number_of_Periods__c":12,
            "LLC_BI__Spread_Statement_Type__c":"a191b0000003gVZAAY",
            "LLC_BI__Spread_Statement_Type__r":{  
               "LLC_BI__Bundle__c":"a1G1b000000oreiEAA",
               "LLC_BI__Type__c":"UCA Cash Flow",
               "Id":"a191b0000003gVZAAY",
               "LLC_BI__Bundle__r":{  
                  "LLC_BI__Relationship__c":"0011b00000Z6Pa8AAF",
                  "Id":"a1G1b000000oreiEAA"
               }
            }
         }
      ]
   }
}

After I run JSON.deserialize(jsonString, Map<Id, MapWrapper>.class) I get

{0011b00000Z6Pa8AAF = MapWrapper:[  
      entity=Account:{  
         Id=0011b00000Z6Pa8AAF,
         Name=Mohyee Holdings LLC
      },
      periodList=(LLC_BI__Spread_Statement_Period__c:{  
         LLC_BI__Spread_Statement_Type__c=a191b0000003gVZAAY,
         Id=a141b0000006lntAAA,
         LLC_BI__Statement_Date__c=2018-12-31 00:00:00,
         LLC_BI__Number_of_Periods__c=12.0,
         LLC_BI__Source__c=Tax Return}
      )
   ]
}

Note that the related sObject LLC_BI__Spread_Statement_Type__r is no longer included in the LLC_BI__Spread_Statement_Period__c object after deserializing.

Here's the wrapper class that JSON.deserialize() is casting to:

public class MapWrapper{
    @AuraEnabled public Account entity{get;set;}
    @AuraEnabled public list<LLC_BI__Spread_Statement_Period__c> periodList {get;set;}      
}