Visa refused with a reason of being a threat to public health

My parents who are living in India have applied for the Schengen tourist visa (Family / Friends) at the Consulate of Germany based in Mumbai.

After 2 weeks, we received an e-mail from the consulate to submit the pending document i.e. “Updated Vaccination Certificate with scannable QR-Code (Please self-check the QR-Code before sending the certificate).”

I self-checked the document by scanning the QR code which was not easy for me because initially the QR code was not recognized then I tried again to scan it for 30-60 sec which finally was successful, and I could verify the vaccine certificate.

Five days after the submission of the vaccine certificate, my mother’s visa was approved while my father’s visa was surprisingly refused stating the following reason "one or more Member States consider you to be a threat to public health as defined in point 21 of Article 2 of Regulation (EU) No 2016/399 (Schengen Borders Code)."

According to point 21 of Article 2 of Regulation (EU) No 2016/399 ‘threat to public health’ means any disease with epidemic potential as defined by the International Health Regulations of the World Health Organization and other infectious diseases or contagious parasitic diseases if they are the subject of protection provisions applying to nationals of the Member States. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:02016R0399-20190611&from=EN

The online explanation of the above statement is mentioned below: https://accra.diplo.de/blob/1120620/580d186ac10c2ade9c0f2aa333861abc/04-11-2020-schengen-visa-refusal-data.pdf

Please note that both - my mother and father- are fully vaccinated with COVISHIELD which is acceptable by the German Paul Ehrlich Institute. Therefore, I am puzzled that why my father’s visa was rejected. But my Mother's visa was accepted.

My question is – Can the Schengen visa be rejected if the QR code of the Vaccine Certificate reflects trouble in scanning? Has anyone faced a similar issue?