>ID field is double with 0 decimals

A id with a double float field ? Why ?
It appeals to to you to play with the fire.

>Order field is Integer

Well.

you can compare your code with this code ? You can repro the error ?

CREATE CURSOR C1 (id B(0),other_id I)
INSERT INTO C1 VALUES (0,0)
INSERT INTO C1 VALUES (1,2)
INSERT INTO C1 VALUES (2,1)
INSERT INTO C1 VALUES (2,7)

CREATE CURSOR C2 (id B(0),other_id I)
INSERT INTO C2 VALUES (0,0)
INSERT INTO C2 VALUES (1,2)
INSERT INTO C2 VALUES (2,3)
INSERT INTO C2 VALUES (2,7)

SYS(3054,11)
SELECT * FROM C1;
WHERE STR(id, 19,0) + STR(other_id,5,0) IN ;
(SELECT MIN(STR(id, 19,0) + STR(other_id,5,0)) FROM C1 GROUP BY id)

Fabio