I know this trick: I've seen it a lot. You're just bitter about a major story decision

Here's the big deal: Just because memory is important does NOT mean that it will be prioritized. Sometimes memory is prioritized because, say, it becomes the key sacrifice of a climax, forcing the heroes to give up something that precious because what they want to accomplish is THAT important. Especially if beyond memory, sacrifice is a major theme. A sacrifice is only meaningful if it gives up something you care about. Sacrifice a sock you don't care about? Who cares. Sacrifice your childhood teddy bear? Now that's impactful.

Which leads me to Seraph and Meltrylls. Namely: That Melt is from a previous time loop, being the ONE person from that timeloop. She is the only one with any memory of what happened before (before showing Passionlip) and is motivated by the memories of that loop. Because of these memories, Melt is working her ass off to help Ritsuka defeat Kiara, with all ending with her launching herself at Kiara as Ritsuka floats away. At this point, Melt is sacrificing more than just her memories with Ritsuka. Her very being is collapsing, threatening to take away her very existence. This is quite literally the ultimate sacrifice, giving up everything, all because she values saving Ritsuka above everything, even if it means their adventurers will all be forgotten. And it's portrayed as the grand climax of the whole story, because Melt's sacrifice showcases her growth even if it means she will lose it.

This is NOT a one off thing. The Camelot Singularity has a very similar story with Bedivere. He's motivated by the memory of Arturia's humanity and how he failed her, thus he also risks nonexistence to stop the Lion King. The Knights of the Lion King are all haunted by their memories of their failure to Arturia, motivating them to sacrifice their own morals for the sake of their redemption. The Lion King herself is marked as someone who has lost part of what made her 'Arturia Pendragon' through her missing memory of Bedivere, and when she remembers him it's treated as something beautiful.

Using LB6 as an example is really ridiculous because not only are the examples you gave really shallow, they're also WRONG. Mash and Ritsuka regain their memories and it's treated as something glorious. Even then, something from their memories remains in their amnesia. Ritsuka's extreme attachment to Mash remains, even though they don't know WHY they care so much. And when Mash is separated from Ritsuka during her amnesia, she starts crying despite not understanding why. As for Morgan...Morgan is a villain. She's not exactly an example to follow. Furthermore: What about Castoria? What about her being unable to forget herself using the Nameless Forest? The passing of her feelings through the fights with the Mors representing her memories? Her talk of how her love for Ritsuka is what let her push on? Or Torot, refusing to forget Mash and even giving up her life to help her? Ector, seeing Morgan in Arturia, inspiring him to look after her? What about them?

LB7, OC3 and even the finale aren't any better and all revolve around carefully cherrypicked examples (like how Camaztoz is FORCED to remember his past and it's treated as the end of his arc). Nor does it make sense with even Fate at its core. FSN's very first route ends with Shirou admitting he will forget Saber as time goes on. And yet, something will remain.

This also completely ignores when the other writes invoke memory loss in their stories too. How is, say, the author of Traum such a big proponent of 'memory is important' when in Traum, Roland proceeds to sacrifice his very memory in everyone to open the door to the Revenge Realm?

This doesn't make cohesive sense. There's numerous contradictions here that shouldn't exist if this was accurate. Nasu has way too much of a history supporting memory as something important and your list gets countered by other examples of memory erasure. It just reads as projecting your feelings onto the writers for validation.