Selecting a brush surface that is flush with other brush surfaces selects all of them after Building Geometry

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.

1. Open QAGame in UE4Editor
2. Open attached text file and copy/paste into default level
3. Select Build
4. Click the top surfaces of each brush

Result: Selecting a brush surface that is flush with other brush surfaces selects all of them.

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