Same experience here for my better half. Stick with the regular vaccine would be my advice, but who knows why ?
Most all know they're infecting you with a very mild case of the flu so you can naturally create antibodies. Perhaps that stronger Senior vaccine was just a bit less mild than intended.
Life's a crapshoot anyway, isn't it ?
Same for me (78)...Doc gave me a normal dose instead of the double, and an anti pneumonia shot...said my immune system is good....first time I had a flue shot as far as I remember.