Issue with the definition of the field selection for a customer account group

Hello everyone.

I'm trying to define the field Transportation zone as mandatory for the standard customer account group 0001 (sold-to-party). I've made this definition at the following levels: Define account groups and field selection for customers (Transaction OBD2) and Define transaction-dependent screen layout (Transaction OB20).

Yet, when I try to change the data regarding a customer belonging to that account group, the field is only displayed.

What's curious, is that when I define that field as suppressed, the field is indeed suppressed.

What's happening?