Help with record pick field

When you open the field settings of 'Diagnosis - DX Code', there should be a link to view the relationship that field is a part of. Can you open that and see if there is a field listed as a 'Reference Proxy'? 

With relationships - there is the 'Related' field that directly links your two tables together, its the foreign key in your child table. You can also set another field as a 'Proxy' - where it will act as a dropdown for users on forms or grid edits. It sounds like you might have lost the proxy field that may have previously been the DX Code. 

If you click into the 'Related' field in your relationship - there is a dropdown to set a proxy field, I would suggest picking from that dropdown the DX code value and that will convert it to a dropdown where your users will select the DX code which would then pull in the associated lookup description.