"SELECT ... WITH LOCK" inside Execute Block corrupts data, even subsequent ROLLBACK does not help.
In the following script, the first select will show

SELECT * FROM T;
A B
==================== ===========
aaaa 1
bbbb 2
cccc 3

while the subsequent selects (before and after Rollback) will show
SELECT * FROM T;
A B
==================== ===========
                               0
                               0
                               0


--------------------
CREATE DATABASE 'C:\TESTDB.FDB';
CREATE TABLE T (A VARCHAR(20), B INTEGER);
INSERT INTO T(A,B) VALUES('aaaa',1);
INSERT INTO T(A,B) VALUES('bbbb',2);
INSERT INTO T(A,B) VALUES('cccc',3);
COMMIT;

SELECT * FROM T;

SET TERM ^;
EXECUTE BLOCK AS
DECLARE I INTEGER;
BEGIN
  FOR SELECT 1 FROM T WITH LOCK INTO :I DO I=I;
END^
SET TERM ;^

SELECT * FROM T;

ROLLBACK;

SELECT * FROM T;

DROP DATABASE;
--------------------