I felt like some background story was missing and I was confused, so I did a bit of searching. I found that this volume is a sequel to a one shot called Mellow Book which is actually chp 4 . It was part of this BL anthology. / https://www.mangago.zone/read-manga/nikutaiha_bl_anthology
I recommend reading Chp 4 first, then Chp 1 onwards.