[SalesForce] Map in AuraEnabled now failing in Spring 19 with “Value provided is invalid for action parameter”

Seems Spring 19 wants stricter typing for aura attributes, but I'm not finding any best practices on how to approach this.

This use to work in Winter 19:

MainController.js

let myData = {
  parentId: "123abc",
  sourceData: [
    {
      childId: "1",
      Some_Bool_Field__c: true
    },
    {
      childId: "2",
      Some_Bool_Field__c: true
    }
  ]
}
helper.engineService(component).sendMyData(
  myData
)

EngineService.cmp

<aura:method name="sendMyData" action="{! c.handleSendMyData }">
  <aura:attribute name="data" type="Object"/>
</aura:method>

EngineServiceController.js

handleSendMyData : function(component, event, helper) {
  var params = event.getParam("arguments");
  var action = component.get("c.goGetMyData");
  action.setStorable();
  action.setParams({
    data : params.data
  });
  helper.dispatchAction(component, action, params); //enqueues action
},

Server

@AuraEnabled
public static Map<String, Object> goGetMyData(Map<String, Object> data) {
  // In Spring 19, we don't even see this next line
  System.debug('appendEngineCache data: '+data); 

  // winter 19 use to parse these correctly
  Id recordId = (Id)data.get('parentId');
  List<Children__c> someChildren = (List<Children__c>)data.get('sourceData');

  // more
}

In Spring 19, I'm not seeing any serverside debugs other than enter and immediately exit Aura with this client-side error:

"Value provided is invalid for action parameter 'data' of type 'Map'"

Here's what I've tried so far with no success:

• change aura attribute type Object to Map (with and w/o default="{}").
• change serverside argument from Map<String, Object> to Object and it gacks

Here's what works, but not ideal (so I'm looking for alternatives)

• Use JSON.stringify(data) and changing apex argument to String then use Map<String, Object> parsedData = (Map<String, Object>)JSON.deserializedUntyped(data) inside the method.
• Then, continue to use parsedData in any downstream Apex.

Anyone found any alternatives without that additional type coercion overhead?

EDIT: Possible solution until more info?

Given that existing unit / integration tests sometimes need to call these methods (with Map<String, Object> parameters), this is the least path of resistance I've found is to overload existing method AND use JSON.stringify at some point in clientside.

Client

action.setParams({
  data : JSON.stringify(params.data)
});

Server

@AuraEnabled
public static Map<String, Object> goGetMyData(String data) {
  return goGetMyData((Map<String, Object>)JSON.deserializeUntyped(data));
}

@TestVisible
private static Map<String, Object> goGetMyData(Map<String, Object> data) {
  // all existing code
  // but @TestVisible added and switched over to private so aura component doesn't find the wrong one.
}