Try
SELECT MAX("fld0"), fld0 FROM table EXCEPT SELECT MAX("fld0"), fld0 FROM table where ("fld1" = 'val1' or "fld1" = 'val2' or "fld1" = 'val3');
and see if that gives any clue as to what is going on.
Try
SELECT MAX("fld0"), fld0 FROM table EXCEPT SELECT MAX("fld0"), fld0 FROM table where ("fld1" = 'val1' or "fld1" = 'val2' or "fld1" = 'val3');
and see if that gives any clue as to what is going on.