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Mm. That may fit for Darth Vader/R2-D2, but it doesn't fit The Seven Deadly Sins. Apparently I needed to ask a slightly different question.
Forgotten First Meeting almost applies, except that it seems like that's meant to be specific to the romance genre. I'm thinking of something more like "Forgotten Personal History".
Like, in 7DS, one of the main characters is Elizabeth. She is one of the three Princesses of Liones. Her dad is the King of Liones. The Seven Deadly Sins were basically a Special Forces group who worked for the King of Liones. As the 1st season progresses and the Sins slowly accumulate, Elizabeth is introduced to all of them as a princess of Liones.
But it's not until the 3rd season where they all have that collective moment of "Oh, you mean that Elizabeth, princess of Liones!" because they all knew her as a kid, since they worked for her dad. They just somehow managed to not put two-and-two together until the middle of the 3rd season.
Another character, Gowther, doesn't recognize Meliodas or Ban. This is despite the fact that both Ban and Meliodas are shown with their helmets off in Gowther's presence in flashback scenes. (But he does recognize King.)
As I say, the series is rife with this, and it's never explained or even acknowledged.
I got dozens of friends / and the fun never ends / that is as long as I'm buying
Writing by the Seat of Your Pants, but that's trivia and it needs authorial confirmation.
Continuity Drift, Retcon, Expansion Pack Past and Series Continuity Error don't, AFAIK.
Edited by WarriorsGate
In the early parts of the series, two characters meet. The characters act like this is their first meeting. The story treats it like its their first meeting. The story continues to treat these characters as though they have no history at all prior to meeting each other in the early part of the series.
Then later, flashbacks are shown which reveal that the characters actually do know one another, and at least one of the characters (if not both) should recognize the other. Sometimes accompanied by an "Oh yeah, that was you!" moment.
I'm thinking specifically of The Seven Deadly Sins, which is filled with this sort of thing. Nobody seems to remember one another until they are introduced to each other on screen.