Are you using a cylinder? Because if you are, you should know that bsp cylinders in unreal automatically place their own vertices off grid, which might cause issues later. i know because i had a bunch of those in my level, and after a while all of the bsp:s in the entire level completely flipped out. if you want to use that shape, i would suggest you use a cube instead, and extrude two faces on opposite ends of the cube and then shape the cube into a cylinder instead. Then it will definitely stay on the grid