lightning:recordForm with lookup to Account Object does not work - Bug Identified (I believe)

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >

    <aura:attribute name="contactfield" type="String[]" default="LastName,AccountId"/>    
    <Lightning:Card class="slds-card__body slds-card__body_inner" iconname="Standard:Contact"
                    title="quick contact">
    <lightning:recordForm aura:id="brokerForm"
                                recordId="{!v.recordId}"
                                objectApiName="Contact"
                                fields="{!v.contactfield}"
                                mode="edit"/>
    </Lightning:Card>
</aura:component>

This works for me. May be this recordId="{!v.theId}" is creating peoblem for you. Are you getting any error while adding component to layout?

Update:-

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >

<aura:attribute name="Branchfield" type="String[]" default="Name,ACCOUNTCHILD__c,Account__c"/>    
<Lightning:Card class="slds-card__body slds-card__body_inner" iconname="Standard:Contact"
                title="quick Branch">
<lightning:recordForm aura:id="brokerForm"
                            recordId="a0T7F000006EEbPUAW"
                            objectApiName="Branch__c"
                            fields="{!v.Branchfield}"
                            mode="edit"/>
</Lightning:Card>

Here i tried with Lightning tab on custom object having a standard object and another with custom object lookup. Is that matching with you what you want?

Update:-

I tried with custom Object having a namespace and working fine:-

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >

<aura:attribute name="Branchfield" type="list" default="['Name','sa_nket__ACCOUNTCHILD__c','sa_nket__Account__c']"/>    
<Lightning:Card class="slds-card__body slds-card__body_inner" iconname="Standard:Contact"
                title="quick Branch">
<lightning:recordForm aura:id="brokerForm"
                            recordId="a0T7F000006EEbP"
                            objectApiName="sa_nket__Branch__c"
                            fields="{!v.Branchfield}"
                            mode="edit"/>
</Lightning:Card>