innodb , mysql Indexes difference when adding index
Try a no-op ALTER:

ALTER TABLE tablename ENGINE=InnoDB;


(Suggested in this forum.)

(Edits)

I do not know why the problem happened.

Here is another 'fix' that might work:

CREATE TABLE b_new LIKE b;
INSERT INTO b_new
    SELECT * FROM b;
RENAME TABLE b TO b_old, b_new TO b;
DROP TABLE b_old;