According to my understanding the auditors should be independent and a rotation system should be in place.
The Swiss State Secretariat for Economic Affairs, SECO (accreditation body) confirms that only in EN 9104:2013 / EN 9100, EN9110 and EN 9120, certification companies are obliged under chapter 8.3.8 to change auditors after 2 consecutively certification cycles.
For ISO 9001, there is no obligation to change an auditor after he or she has conducted the audit in the same company 2 or more times.
The negative impact in terms of quality is obvious.
The question for me is why ISO has not stipulated rotation rules for auditors in ISO 9001?