If they already received their ETAs then they should proceed as if there is no issue.
It’s possible they will get to Canada and they will be asked about their status. But this was an error made by the government so it’s hard to see how they were responsible for it. It’s unlikely they will be denied entry because they were wrongly issued ETAs (though it’s definitely a possiblity).
It is likely that this mistake will not be made again in the future, so next should definitely renounce their PR statuses before they apply for their next ETAs.