I've had the rejection moodlet from a negative milestone on several sims over the years, but I just realised yesterday what might be the reason for it.
I played a sim that got a wish to flirt with another sim, of course another sim got involved in the conversation just as my sim started to flirt, so it because a negative action/ moodlet for it. Basically he got rejected and it got awkward.
A sim day or two later he got the remembering rejection moodlet, and I checked his milestones but the few he has were all positive. Then I realised the moodlet must be from the flirt rejection he had, I had only played him for a short while until then and this was one of his first interactions with other sims.
I don't even know if there is a moodlet for rejections, I think not, so it's probably just that the moodlet is from rejection but there is no corresponding milestone that we can see (but there might be one in the background), or the text on the moodlet is wrong when it refers to a milestone.