when you use Boolean/ProBoolean it tends to change the smoothing groups and this is more than likely an affect of multiple smoothing groups. It may also be due to the vertices on this face being slightly out of alignment.
I believe the fix here would be to simply set the same smoothing group to be the sme for all the polygon faces and use the Planar XYZ (depending on your angle) to align all the vertices.
Here is a video I recorded a few months ago which takes you through some of the steps when I use the boolean tool and cleaning up the mesh. Hope you find it useful!
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Technical 3D Graphic Artist
Autodesk 3dsMax 2015 - Service Pack 4
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