This issue was reported by internal UT developers.
When selecting the surface of a brush that share flush surfaces with other brushes, all surfaces are selected. This seems to only occur after selecting Build, as auto build doesn't reproduce this.
Result: Selecting a brush surface that is flush with other brush surfaces selects all of them.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-35482 in the post.