Deja Vu is a "save error" where we basically skip RAM and throw something straight to the hard drive. So we "remember" as we experience it, but can never get ahead of the present moment to "remember" the future.

Because it's still just the present. We're just experiencing a very short term glitch which is relatively normal, or accidentally inhaled a bunch ether or something similar and shits gonna be weird for 2-3 minutes straight before it wears off and the brain recovers.

Like, there's not just short/long term memory. There's a bunch of different types, but the "now" experience is it's own type of memory and last fractions of a second. Break that even momentarily and you experience deja vu because everything just goes straight to the next level of memory. Which is working memory and cover seconds to minutes.

Déjà vu is when you think you've seen/done something before but haven't. It's not a flashback at all.

Flashbacks aren't always traumatic

I'm missing something though. I feel that a deja vu has a different quality. A feeling of familiarity with something that you're not supposed to be familiar with, almost predictive not quite a prediction. While having a flashback, traumatic or not, is a familiarity with something that you're actually already familiar with. And it invokes different emotions of anger, fear, disappointment, joy, etc.

A déjà-vu isn't a flashback at all.