transaction_registry.begin_timestamp is wrong for explicit transactions

create or replace table t1 (

    x int(11) default null,

    row_start bigint(20) unsigned generated always as row start invisible,

    row_end bigint(20) unsigned generated always as row end invisible,

    period for system_time (row_start, row_end)

) engine=innodb with system versioning;

begin;

set @ts1= now(6);

select sleep(1);

insert into t1 values (1);

commit;

select row_start from t1 into @trx_id;

select trt_begin_ts(@trx_id) <= @ts1;