Wrong behavior when insert default value

[winoros]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a int default 1, b int default 2);
insert into t (b) values(default);

2. What did you expect to see?

One row which a is 1, b is 2.

3. What did you see instead?

One row which a is 1, b is 1.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

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