Hm, not so much losing their memories, but more so being ignorant of their past incarnations. I made an OC that was originally just a character in my main OC's fictional story, but he then made the fictional OC and their world real. But, prior to that, the world has been living in a constant loop of the same story, with each iteration being unaware of the repeating events.

However, when the main OC gave the fictional OC and his world autonomy, they then gave the fictional OC the memories of their past incarnations a little later, and explained their existence. However, during the time where the fictional OC was suspicious, he became slightly frenzied and extremely distributed. He showcased traits that were far from inline with his character.

Here's the character: Leo Valor

Malware is a Theodore from Downfall that manage to get his happy ending but messed with his matter which ended up sending him to the Null Zone where he loses all memory except for an anger towards everyone.

.Tsuqiya is a student who loses half of his heart (Kingdom Hearts style) when a complex dark ritual splits his inner duality.

What remains has been sealed in a locket-pendant he cannot remove.

.While he is capable of emotions, he feels only half of it, which makes him seem uncaring.

He forgot the names & faces of his parents, sisters & childhood sweetheart.

(Previous to that, he ran away from home, thus why they aren't present.)

.When he activates his transformation, his sealed heart is temporarily liberated.

Which makes him wildly untamed & undisciplined, having near-constant emotional bursts.

(The least bad factor in that situation is that his polar opposite attitudes help conceal his identity from people, which he's somehow grateful for.)

.When he regains his memories in 3/4 of the story, it's not pleasant. It's painful & coma-inducing.

Eventually, he wakes up, a complete being & even more powerful than before.

But his new condition leaves him unbalanced for some months, and he has to rehabilitate himself.

Jay again. He went made and lost his memories and old personality because of his obsession with human pop culture. This led to him being the mad god of today.

I have an OC who reincarnates but always loses her past lives memories so she has to rebuild herself every life.