Bug #30847 Critical error with a stored function on WHERE

Description:
First check my stored function:

FUNCTION `sigmuestra`(muestra VARCHAR(15)) RETURNS varchar(15) CHARSET latin1
BEGIN
        DECLARE I INT;
        DECLARE car INT;
        DECLARE resp VARCHAR(15);
        DECLARE bandera BOOL;
        SET I = LENGTH(muestra);
        SET resp = muestra;
        Buscar: LOOP
            SET car = ASCII(MID(resp,I,1));
            IF car >47 and car < 57 THEN
                    set car=car+1;
                    SET bandera = true;
            elseif car = 57 THEN
                    set car=48;
                    SET bandera = false;
            END IF;
            SET resp = CONCAT(LEFT(resp,I-1), CHAR(car), right(resp,LENGTH(resp)-I));
        SET I = I - 1;
        IF I = 0 OR bandera THEN LEAVE Buscar; END IF;
        ITERATE Buscar;
        END LOOP Buscar;
        RETURN resp;
END

Now check this other SELECT statements and returns... they are logically impossible. But it work properly in other computers:

table muestra:

Nombre
A01b
A02b
A04b
A06b
A07b
A09b

I will use "count(*)" in SELECT statement... but the problem is the same with "*"

1- select count(*) from muestra as m, muestra as n
     return: 36

2-select count(*) from muestra as m, muestra as n WHERE sigmuestra(m.nombre) =n.nombre;
     return: 0

3-select count(*) from muestra as m, muestra as n WHERE sigmuestra(m.nombre) <> n.nombre;
     return: 34

My function is working well... look:

SELECT sigmuestra(m.nombre), n.nombre FROM muestra as m, muestra as n;

Return

+----------------------+--------+
| sigmuestra(m.nombre) | nombre |
+----------------------+--------+
| A02b                 | A01b   |
| A03b                 | A01b   |
| A05b                 | A01b   |
| A07b                 | A01b   |
| A08b                 | A01b   |
| A10b                 | A01b   |
| A02b                 | A02b   |
| A03b                 | A02b   |
| A05b                 | A02b   |
| A07b                 | A02b   |
| A08b                 | A02b   |
| A10b                 | A02b   |
| A02b                 | A04b   |
| A03b                 | A04b   |
| A05b                 | A04b   |
| A07b                 | A04b   |
| A08b                 | A04b   |
| A10b                 | A04b   |
| A02b                 | A06b   |
| A03b                 | A06b   |
| A05b                 | A06b   |
| A07b                 | A06b   |
| A08b                 | A06b   |
| A10b                 | A06b   |
| A02b                 | A07b   |
| A03b                 | A07b   |
| A05b                 | A07b   |
| A07b                 | A07b   |
| A08b                 | A07b   |
| A10b                 | A07b   |
| A02b                 | A09b   |
| A03b                 | A09b   |
| A05b                 | A09b   |
| A07b                 | A09b   |
| A08b                 | A09b   |
| A10b                 | A09b   |
+----------------------+--------+

How to repeat:
It doesn't occurs in other computer :P

Suggested fix:
Kill me please!!!

Thank you for a problem report. Please, send the results of:

show variables like 'char%';

from the machines where your function "works" and "does not work".

No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".