Hi! I get this too! Feel free to message me any timeHey there! I am also going through this after a few months of intense health anxiety. I haven't found a concrete answer but I've met many others online who experience this.
Best I can tell, DPDR alters the function of some parts of the brain, causing overactive alpha wave production. This is important, because alpha waves are produced when we dream! So, essentially, DPDR causes our brains to produce some mild dreamlike feelings while we are awake. This could be triggered by any random thing we see and will pull out these eerily nostalgic feelings, same as what happens we we dream.
Hope this helps!