... upon replicating online ALTER

When an online event is applied and slave_exec_mode is idempotent,
Write_rows_log_event::do_before_row_operations had reset
thd->lex->sql_command to SQLCOM_REPLACE.

This led to that a statement was detected as a row-type during binlogging,
and was logged as not standalone.

So the corresponding Gtid_log_event, when applied on replica, did not exit
early and created a new PSI transaction. Hence the difference with
non-online ALTER.