Hi LoveSofa,
I’m looking into this issue and have been getting different results. I am currently testing in 4.7.3 with two BSPs, one box, one cylinder. The box is subtractive and the cylinder is additive, the section cuts out, but I am not getting a square like that.
Could you tell me the exact dimensions of the objects and the number of sides the cylinder has? Which version of UE4 are you using? Also, does this only happen with those two shapes or do other brushes do this as well? Any additional information may help in the investigation.
Cheers!