Cannot update lookup field!

Cannot update lookup field!

suddenly I have lost the ability to edit the value in a lookup field (Product_ID) in a particular form (Account_Products)

https://creator.zoho.com/hayley.rees/admin/Account_Products/record-edit/Account_Products_Validation/483991000005298057/

I can still bulk edit it, but if I attempt to change it directly and click update, the change doesn't take effect!

Why? there is nothing in the on update or any other part of the deluge script to prevent this change and even if there was, it would error... instead, I get success message but the change doesn't take effect for that field... I can still change the other fields in the form!

Please help!!