Hello,
Its strange that in Prod behavior is different. You can make tax code field mandatory in info record by: SPRO-MM-Purchasing-Purchasing info record-Define screen layout. if this is for creation you need to select field selection ME11 and have to go to conditions. There you have to make 'info record' as required field. But please note this will be for all types of info records, which may not be required in your case. So in this case you will have to use user exits to check if tax code field is maintained. For example you might not require tax code in case of standard info records. Contact you ABAP team. They might help you.
Got this thread for config comparison: Tools to Compare SAP SPRO Configuration between 2 instances. See if this helps you.
Check Me14 for info record changes.
Regards..