ssmp | highly optimized message-passing library | Reflection library

I'm working in a dev SQL database within SSMP.

We've got a junction table, jnc_roles_users, which pulls values from two lookup tables: lu_roles and lu_users.

Initially, all but one column in the junction table had the Allow Nulls checkbox checked. I then went into the design editor and checked all the boxes - got the alert that this will affect the two lookup tables, and saved the changes successfully.

Now, I want to switch back to the original table design of nothing allowing nulls except one column. But SSMS is no longer allowing me to uncheck the Allow Nulls box for one of the fields. Ironically, no problem unchecking our 'ID' primary key. It's our 'pseudo' primary key 'roleUserID' that I can't change back. When I try to uncheck 'Allow Nulls' for it and save, I get the alert:

'jnc_roles_users' table - Unable to modify table.
Cannot insert the value NULL into column 'roleUserID', table 'jnc_roles_users'; column does not allow nulls. INSERT fails. The statement has been terminated.

This seems counterintuitive because the column is currently stuck at 'Allow Nulls', yet this popup is telling me it can't accept nulls.

lu_roles and lu_users was saved successfully, but not jnc_roles_users which is the only table which contains 'roleUserID'.

The roleUserID column isn't technically a primary key - but we're considering it 'like' one as it does take unique values that we set. For certain reasons we're considering this column as a sort of primary key - something to do w/ database backups in different locations and how the regular ID pk might get duplicated incorrectly in different environments, so we needed a second pseudo pk of roleUserID.

Any ideas?