Adding a 2nd lookup to an form in the new process UI will cause that form to fail to load every time.
For example, Account form has Primary Contact as a standard lookup. Now add a 2nd lookup to Invoicing Contact & place on the form. Save & Publish and soon as you open a record you get an error message and worse still you now cannot save any changes to the record at all. Only solution I've found so far is to remove the 2nd lookup - all then works fine again.
Roger