Refer to Chapter 23 Translator Notes:
https://mangadex.org/chapter/a98a61b9-de13-4891-aa48-b06eeb189dbd/1
The first 22 chapters are set in the present, while all chapters from chapter 23 should basically be considered a 'flashback' of sorts and are set in the past, giving background to their relationship.
I don't know why the mangaka opted to write it in such a confusing way without spelling it out in the manga or making it clear, but that's just how it is