It’s not clear exactly what happened. Here’s what the FAA is saying:
5. Purpose of the Reexamination. As a result of an investigation by the FAA of the certification activities of Mr. Puehler, the Administrator determined that there is reason to believe that the competency of the airmen examined by Mr. Puehler from the period of October 2008 through December 2019 is in doubt. As a result of the evidence obtained during the investigation, the FAA determined that these airmen will require reexamination of their competency pursuant with 49 U.S.C. § 44709 to ensure safety in air commerce.
If a pilot passed a more advanced rating from a different examiner after “passing” with Mr. Puehler, they are not subject to reexamination. Notable exclusions are airline transport pilots and flight instructors.
The shocking thing is that this guy is a well known master instructor and has been a speaker at FAA sponsored events.