The breaking up didn't make sense at all and kind of ruined the ending?

Ike-sen more than likely ran out of space/time to push out the last two chapters so it was hella rushed. I feel like if one more chapter coulda been squeezed in it would've been better. Andou left 'cause he felt like he was gonna get in the way of Setsu's success despite them being in love. He got himself tangled up with yakuza (Kabu is the wakagashira of the Hashiya and he's actually from another really well put together series of Ike-sen's) and he damn near got Setsu hurt by it as well. Andou knew his love for Setsu was making him make rash decisions so he chose to leave instead of continuing to put them both in danger.