PostgreSQL triggers with LibreOffice Base front-end

I have the following trigger function & trigger in PostgreSQL 12.1:

create or replace function constraint_for_present()
returns trigger
as $$
BEGIN
if
    new.present_status = 'viewing'
    and new.name not in (select viewable_item from sourcing)
then raise exception 'a present_status of "viewing" requires that the viewable item is in sourcing';
end if;
return new;
END;
$$ language plpgsql;

create trigger constraint_for_present
    before insert or update of present_status on viewable_item
    for each row
    execute function constraint_for_present();

These work as expected during data entry in the psql and TablePlus clients. However, the function throws an error when accessing the database via LibreOffice Base:

pq_driver: [PGRES_FATAL_ERROR]ERROR:  relation "sourcing" does not exist  
LINE 2:   and new.name not in (select viewable_item from sourcing)  
    
QUERY:  SELECT new.present_status = 'viewing'  
    and new.name not in (select viewable_item from sourcing)  
CONTEXT:  PL/pgSQL function viewing.constraint_for_present() line 3 at IF  
(caused by statement 'UPDATE "viewing"."viewable_item" SET "present_status" = 'none' WHERE "name" = 'test4'')

In Base I have a simple form set up for the trigger's table, with each foreign-key column set to list box, and the Type of list contents set to Sql (also tried Sql [Native]). The List content of each is (with appropriate table and primary key columns):

select name from viewing.cv_present_status order by name

(This database is using natural keys for now, for organizational political reasons.) The Bound field is set to 0, which is the displayed and primary key column.

So ... 2 questions:

1. Why is this problem happening only in Base, and how might I fix it (or at least better trouble-shoot it)?
2. Since Bound field appears to take only a single integer, does that in effect mean that you can't use list boxes for tables with multi-column primary keys, at least if there is a single displayed column?

In the trigger function, you can fully qualify the table

...
   and new.name not in (select viewable_item from viewing.sourcing)
...