Is this inconsistent treatment of `void` return from sql and plpgsql functions documented?
I don't believe it's documented. The behavior is incorrect or at best inconsistent but maintained probably because of a huge existing user code base that would break if it were corrected. The issue is caused by differences in how returns VOID is coded with SQL vs plpgsql. See: https://stackoverflow.com/questions/8319986/postgresql-functions-returning-void