As I said, the GP knew me mum. I may have been marginal. It was nearly 44 years ago.
I believe there are others who have questioned the accuracy of an Ishihari. But I may very well be wrong. Tell them you want a second opinion. Worst that can happen, you fail again.
I had a similar issue when joining up.
Ishihara said I was red green colourblind - news to me as I never had any issues with colours until you stick those vomit inducing dots in front of me then I can give about 4 possible answers.
Recruiting Sgt looked at the results wasn't happy and then referred me to a consultant who passed me as fine. Ishihara isn't suited to everyone, which is unfortunate when so many take it as the only test.
Police were using City Test when I joined and I recall having an argument with the optician and pulled out Ihshiara and then spent his time flipping through the pages sighing as I gave my selection of answers clearly deciding that I could only see black and white. I pointed out that they wanted City test which he obviously didn't have and basically bullied him into signing me off.