Flashbacks involve a character looking back on something that happened in their OWN past, not a POV change that rewinds time. This is being presented from a non-Hikari POV and is being told as current events, not a memory. The only thing that blurs this slightly is Ayami's last page in Ch 21 (which IMO the story would be better with NO explanation of Ayami's past at that point so that we could discover it all ourselves through the storytelling).
Definition of a Flashback
In literature, a flashback is an occurrence in which a character remembers an earlier event that happened before the current point of the story. The definition of flashback is identical to that of analepsis, which comes from the Greek for “the act of taking up.” There are two types of flashbacks—those that recount events that happened before the story started (external analepsis) and those that take the reader back to an event that already happened but that the character is considering again (internal analepsis).