Check your Workflow scripts for the Defects Module Scripts / Bug_New script/method.
It will likely be running another subroutine called WizardFieldCustAdd. This was generated by the script generator for Add Defect.
There are many lines in this code spelling out behavior of each field on the ''form'' that comes up when you create a new defect (using the SetFieldApp function).
Find your problem field (identified by BG_USER_xx, where xx is an integer), and look at the settings for it. The second true/false value should be ''False''. Check for more than one occurrence of the field - there will be one for each security ''group''.
Save the code and apply changes, then get out of the project and go back in to check it.
It is unexpected, but when you have ''form'' customization dictating how the fields behave on the form, it trumps the ''required'' behavior set on the entity.