>>you can remember some other boolean functions that it cannot return NULL ?
>
>ISBLANK(), ISALPHA(), e.t.c.

But why those of the team want to check what they are not able
to check, and to create tons of cases where control becomes a bug?

Of course,
with an UDF they have gone to crisis, and therefore they have disabled the check:

ON ERROR
CLEAR
CREATE CURSOR testBinary (aa i NULL)

INDEX ON UDF(AA) TAG TBIN2 BINARY

* a UDF remove the nullable check
INDEX ON SAME(ISNULL(AA)) TAG TBIN1 BINARY

* and then fail here
INSERT INTO testBinary VALUES (1)

PROCEDURE UDF(XX)
RETURN XX=0 OR .NULL.

FUNCTION SAME(X)
RETURN m.x

Of course the message is clear, and says that is TBIN2 that fails!!!!