I sometimes remember dreams from easily 15y - 20y ago absolutly vividly, although I have had forgotten them in the mean time. Suddenly, during the day, like in a split second (maybe triggered from a scene or scent) I feel that "feeling" (or more like a superposition of all impression of that dream i had) of a dream long lost and the dream is fully present in that moment and I kind of "remember" it completely. Then it is fading away fast.
Every so often I have these vivid dreams with what feels like long story archs. In my youth / pre 30s more often then now. At the end of my 20s I started to keep a dream diary, not very consistently, but still.
So now, when I have these dream flashbacks, I immeadiately grab this journal or a piece of paper. I write everything down and draw sketches of the scene and really try hard to remember specifics of that dream. And then something really weird happens... i start to remember scences form other forgotten dreams and it feels like I opened a door or pathway in my memory and can look/explore fragments of these dreams. And the harder I try the better results I get. It is really an odd experience.