[CMT8.4.1.1209]Nullable and unique option hasn't been worked.

1 There is one table
CREATE TABLE `ab` (
`b` decimal(63,0) DEFAULT NULL,
`a` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`a`),
KEY `b_idx` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='aaaass'
2 do migration from online to online,
change column b to: don't choose nullable; choose unique, input default value to 3, and choose auto_increment.

Actual result:
It will change to
CREATE TABLE "ab"(
"b" numeric(38,0) AUTO_INCREMENT NOT NULL,
"a" integer DEFAULT 0 NOT NULL
)

and the history isn't right.