One possible theory is that Mia had a habit of writing diaries. It seems implied that she attempted to suicide at one point, and could have lost her memories due to a failed attempt. This also explains why she doesn't seem to remember some things and her change in preferences such as coffee etc. Because she doesn't remember them personally, but read them from a third person's perspective.
I also originally thought of this problem with fragments of her memory being an example similar to Kaede (Sakuta's little sister) in Rascal Does Not Dream/Aobuta being two separate people pre-amnesia and post-amnesia due to trauma (though she goes back to pre-amnesia Kaede and post-amnesia one is erased).
If you include that 'diary' angle, like the amnesiac Kaede in Aobuta kept, then perhaps Mia truly has full amnesia and only reconstructs her past from journals, photographs, messages, and other records she kept. Maybe just a smartphone alone would be enough too given the vast amount of data on one's life it retains at any given time. In Taesoo's flashbacks, Mia clearly enjoys reading, and and sometimes such people keep written accounts of their lives, so it’s plausible that diaries and photos are the only concrete fragments she has.
That would explain why Chapter 19’s flashback/memory/dream reads like a dark children’s picture-book, and why small, unrecorded details, like the coffee, the hill, the porridge, her actual feelings for Taesoo, and so on are missing from her head. There are hints for this too. In chapter 1, she calls him “the real Taesoo” at the police station when they first meet again, yet she seems to be learning his character anew through their interactions, revising an initial rich-boy arrogant stereotype into someone kinder and more grounded. If her perception and knowledge came only from snippets of recordings and notes rather than actual lived experiences she could rely on in her memories, it would make sense that her first impressions were incomplete and that she's gradually forming a truer sense of him.
Either way, we still don't have any answers about what happened in that 5-year gap, as well as why she thinks if he learns the truth, he would resent her. Also no explanations of where she's getting her funding given the level of poverty she was experiencing as an adolescent/teen, she's not been shown to do any part-time jobs either. No more details for reasons behind her insomnia that has come up a number of times now, and still nothing on why she said she would leave/disappear after graduation. The mystery and drama thickens.
One interesting bit, since the art here has intentional hints here and there, the imagery of her abandoned home in the very first chapter that taesoo went to when she disappeared has the same monochrome with highlighted red motif and broken glass and blood splatter overlay as chapter 19's end of flashback dream.