If you were logged in you would be able to see more operations.
CREATE TABLE A
ID CHAR(1), NAME VARCHAR(255)
CREATE INDEX IDX_A ON A (ID);
CREATE EXCEPTION EX_PERM 'AAA';
INSERT INTO A (ID) VALUES ('1');
SELECT * FROM A WHERE ID = '1'
-- record is present
EXECUTE BLOCK AS
UPDATE A SET NAME = 'xxx';
UPDATE A SET ID = '2';
SELECT * FROM A -- record with ID = 1 is present
SELECT * FROM A WHERE ID = '1' -- no record is returned
COMMIT; -- corruption - index is missed entries
I tested it with HEAD only but i think this is very old bug