Hello blindfoldjump,

it seems that your undelying surface (cylinder)? had start/end meeting (360°) in that edge. So bool will make two faces here. You can seperate the solid in faces, delete them, join upper edges, and lower edges , create Network with the loop of edges to patch the hole, and join everything to a solid. Nevertheless the edge isn't really a Problem.

Hope it works for you!

Kind regards
Karsten