What Criteria to use in Process Builder to check for NULL reference?

Try using the "Formula evaluates to true" option to check for null with the following formula:

[Account].Account_Parent_Tiered_OBJ__c  = null

Always check your conditions - and than check them again, or just rewrite the process.

My error resided in the simple solution: The condition #3 that I used, was wrong. Instead of asking for: Account->Account_Parent_tiered_Obj__c - I had to filter by: Account->Parent->Account_Parent_tiered_Obj__c

The lesson really is to recheck your criteria.

I faced the exact same problem with Process Builder and then the following solution helped.

Whenever we are referring a related record in Process Builder we should put a NULL value check for that field as the first condition in filter criteria.

Then use "Customize the Logic" e.g. (1 AND 2 OR 3) So if 1 fails the process will not go further.

Regards, Rosh