Error while adding TTL to a column which refers EPHEMERAL column

[chrootweili]

ClickHouse version: v22.3.9.19-lts / v22.6.4.35-stable / v22.7.1.2484-stable

drop table if exists test_1;

create table test_1
(
	a String EPHEMERAL '',
	b String DEFAULT concat(a, '-xx')
) Engine = MergeTree() order by tuple();

insert into test_1(a) values('a1');

alter table test_1 modify column b String DEFAULT concat(a, '-xx') TTL now() + toIntervalMonth(3);

Received exception from server (version 22.3.9):
Code: 341. DB::Exception: Received from 127.0.0.1:9000. DB::Exception: Exception happened during execution of mutation 'mutation_2.txt' with part 'all_1_1_0' reason: 'Code: 47. DB::Exception: Missing columns: 'a' while processing query: '_CAST(concat(a, '-xx'), 'String')', required columns: 'a' 'a'. (UNKNOWN_IDENTIFIER) (version 22.3.9.19 (official build))'. This error maybe retryable or not. In case of unretryable error, mutation can be killed with KILL MUTATION query. (UNFINISHED)