An update deadlock bug

Hello everyone:

Recently I found a parallel update bug，The steps to recurrence are as follows:
1,create table t1(a int, b varchar(20));
   insert into t1 values(generate_series(1,5), 'abc');
2, use pgbench to run parallel update:
    pgbench -U postgres -p 5432 postgres -n -r -c 10 -j 1 -T 100 -P 1 -f test.sql

    test.sql:  update t1 set a = 123 where b = 'abc';
3, We will find "deadlock detected" in log file or terminal.

I want to figure out that if every update use sequence scan ,why deadlock happened?

I look forward to receiving your reply.

Thank you very much!