This is caused by temporal AA and not easily fixable.
When adding a bsp Geometry object into the world the edges flicker. The effect can sometimes be seen better if you change the tessellation of an object that has it.
Observed on border edges of a selected Additive brush.
When using with a Subtractive brush, effect is observed where a Subtractive brush intersects and Additive brush.
Result: Heavy flickering
Expected No flickering should occur.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-30764 in the post.