It appears that instead of setting a null property to null when returning an sObject via Apex, the property simply isn't set at all and creates odd problems in the UI

nullSObjectPropertyBugDemo.app

<aura:application controller="ccmt.NullSObjectPropertyBugDemoController">
	<aura:attribute name="thisLead" type="Lead"
                    default="{ 'sobjectType': 'Lead' }"/>
    <aura:attribute name="leads" type="Lead[]"/>
    
    <!-- Event handlers -->
    <aura:handler name="init" value="{!this}" action="{!c.handleInit}"/>
    
    <!-- App view header -->
    <h1><code>null</code> sObject Property Bug Demo</h1>
    
    <!-- List leads -->
    <ul>
        <aura:iteration items="{!v.leads}" var="lead">
            <li>
                <a onclick="{!c.selectLead}"
                   href="#" data-record-id="{!lead.Id}">{!lead.Name}</a>
            </li>
        </aura:iteration>
    </ul>
                                            
    <!-- Edit this lead -->
    <ui:inputText label="Lead ID" value="{!v.thisLead.Id}" disabled="true"/>
    <ui:inputText label="First Name" value="{!v.thisLead.FirstName}"/>
    <ui:inputText label="Last Name" value="{!v.thisLead.LastName}"/>
    <ui:inputText label="Company" value="{!v.thisLead.Company}"/>
    <ui:inputPhone label="Phone" value="{!v.thisLead.Phone}"/>
    <ui:inputEmail label="Email" value="{!v.thisLead.Email}"/>
    <ui:button label="Save" press="{!c.save}"/>
    <ui:button label="New" press="{!c.reset}"/>
</aura:application>

NullSObjectPropertyBugDemoController.cls

public class NullSObjectPropertyBugDemoController {
    
    @AuraEnabled
    public static Id editLead(Lead newLead) {
        Id newLeadId = null;
        
        try {
            upsert newLead;
            newLeadId = newLead.Id;
        }
        catch (System.Exception caught) {
            ApexPages.addMessages(caught);
        }
        
        return newLeadId;
    }
    
    @AuraEnabled
    public static Lead getLead(Id leadId) {
        List<Lead> matchingLeads = [
            SELECT Id,
            	Company,
            	Email,
            	FirstName,
            	LastName,
            	Phone
            FROM Lead
            WHERE Id = :leadId
        ];
        
        return matchingLeads.isEmpty()
            ? null : matchingLeads.get(0);
    }
	
    @AuraEnabled
    public static List<Lead> getLeads() {
        return [
            SELECT Id, Name
            FROM Lead
            ORDER BY LastModifiedDate DESC
            LIMIT 10
        ];
    }
    
    @AuraEnabled
    public static Lead newLead() {
        return (Lead)Lead.sObjectType.newSObject(null, true);
    }
}

nullSObjectPropertyBugDemoController.js

({
    handleInit : function(component, event, helper) {
        helper.initThisLead(component);
        helper.refreshLeads(component);
    },
	reset : function(component, event, helper) {
        helper.initThisLead(component);
	},
	save : function(component, event, helper) {
		var self = this;  // safe reference
        
        // Create an action to save the values entered
        // for thisLead, which will result in an insert
        // or update operation in Salesforce depending
        // on whether the lead is new or pre-existing
        var saveAction = component.get("c.editLead");
        saveAction.setParams({
            "newLead": component.get("v.thisLead")
        });
        saveAction.setCallback(self, function(a) {
            var recordId = a.getReturnValue();
            
            if (recordId != null) {
            	component.set("v.thisLead.Id", recordId);
        		helper.refreshLeads(component);
            }
        });
        
        // Enqueue the action
        $A.enqueueAction(saveAction);
	},
    selectLead : function(component, event, helper) {
        var self = this;  // safe reference
        var recordId = event.target.dataset.recordId;
        
        // Create an action to retrieve the specified
        // Lead, based on the given salesforce.com ID
        var selectAction = component.get("c.getLead");
        selectAction.setParams({
            "leadId": recordId
        });
        selectAction.setCallback(self, function(a) {
            component.set("v.thisLead", a.getReturnValue());
        });
        
        // Enqueue the action
        $A.enqueueAction(selectAction);
    }
})

nullSObjectPropertyBugDemoHelper.js

({
    initThisLead : function(component) {
		var self = this;  // safe reference
        
        // Create an action to set thisLead to a new,
        // fresh Lead object obtained via Apex
        var initAction = component.get("c.newLead");
        initAction.setCallback(self, function(a) {
            component.set("v.thisLead", a.getReturnValue());
        });
        
        // Enqueue the action
        $A.enqueueAction(initAction);
    },
	refreshLeads : function(component) {
		var self = this;  // safe reference
        
        // Create an action to retrieve leads from
        // Salesforce and set the leads attribute to
        // whatever is retrieved
        var refreshAction = component.get("c.getLeads");
        refreshAction.setCallback(self, function(a) {
            component.set("v.leads", a.getReturnValue());
        });
        
        // Enqueue the action
        $A.enqueueAction(refreshAction);
	}
})