Sort:  

I find that if I try to tell them "I'm the one who told you that six months ago", they either don't believe me, or sort of look puzzled and say "I don't remember that". I don't think it's ever earned me any detectable "social credit".

Then you found yourself someone lacking virtue.
You also want "Social Credit" whereas that means nothing to me.

"It is no measure of health to be well adjusted to a profoundly sick society." -Krishnamurti

I'm inclined to believe them, since I've experienced a similar phenomenon myself.

Believe who and what?

When other people tell me that they told me the same thing I think I'm telling them, about six months ago.

Same.