When users are assigned in sequential on-demand review, all users in the review will get notification immediately.
But when the first user completes their part of the review, the second user does not get a notification that will notify them that the content is ready to be reviewed by them.
Such a notification should be there. Otherwise, the second user will not know that it is ready to be reviewed by them. They will know it is ready only if 'Send reminder' is clicked. But this is manual. There should be automated email to the second user when the first user completes review.