We have two custom email fields on our Contact object.  I've created a simple flow triggered by process builder to used a web-to-case email address to find all occurances of that email address in one of those fields.  The flow works as follows

1)  Fast Lookup on Contact using the Web Email address to match against an email field. 
2)  Loop and Assignment elements to count the number of occurances
3) Update element to  write out the number of occurances to the Case record. 

Here's the problem.  This works perfectly if the Fast Lookup is checking the standard Email field on the Contact.  However, if I search on a custom email field, the flow fails:  
 
I have two versions of this flow that are identical except for the field that's being searched.  

(Yes, we're using Person Accounts, but the behaviour is the same if the contact is not a person account.)

Is there any good reason why flow would treat a standard email field differently from a custom email field?

Thank You. 
 

We have two custom email fields on our Contact object.  I've created a simple flow triggered by process builder to used a web-to-case email address to find all occurances of that email address in one of those fields.  The flow works as follows

1)  Fast Lookup on Contact using the Web Email address to match against an email field. 
2)  Loop and Assignment elements to count the number of occurances
3) Update element to  write out the number of occurances to the Case record. 

Here's the problem.  This works perfectly if the Fast Lookup is checking the standard Email field on the Contact.  However, if I search on a custom email field, the flow fails:  
 
I have two versions of this flow that are identical except for the field that's being searched.  

(Yes, we're using Person Accounts, but the behaviour is the same if the contact is not a person account.)

Is there any good reason why flow would treat a standard email field differently from a custom email field?

Thank You.