We just upgraded to 2021.1. Sales is now experiencing this issue:  
 
Open an existing sales order
Cust code field is active
Tab past that field and it is now greyed out.  
Sales wanted to edit that field, but now they are unable to go back and edit.
 
Temp Workaround:  slow down
 
Instead, sales needs the SO-A cust code field to remain active in the sales order so that they can change it like the used to.
 
Please advise if this is due to a new setting (simple fix) or if the cause is something else. I did quickly review the Sales Order defaults and didn't see anything related.  I can also reproduce what is going in the test company.
 
Thank you!
 
Kind regards,
 
Val