Boku dake ga Inai Machi:
https://mangadex.org/title/5049/boku-dake-ga-inai-machi and the spinoff/epilogue https://mangadex.org/title/5049/boku-dake-ga-inai-machi should be merged.
The spinoff is the 9th volume.
http://www.b2c.hachettebookgroup.com/series/erased/?yen (Yen press volumes, also...
@Qelix Could be a mistake, am not affiliated to the group so I can't say anything there tbh. The scanlation group is dead, so this is what we have now.