
Is this a prequel or sequel to Hana to Junketsu??

This is just a spin off, but we might get to see the couple from Hana to Junketsu in extras, who knows? See here http://www.mangago.zone/read-manga/bebe_san_no_koibito/mi/ddc4fe2c6d7f9af299a75c3602aef77f_chapter-1/41/

You’ll find your answer in this chapter: https://toonkor.red/사랑은_환상!_34화.html

Here it is. The translated version. https://www.facebook.com/loveisanillusionmanhwa/
Hara-san's red string is located in his left pinky finger. It disappeared on ch.7 from page 11 to 17. Maybe fate sided with them and erased Hara-san's string so they could be happy together. And the author said so that the two of them would be extremely happy.