Just saw this while trying to fix the geometry of the stuff of a friend just to find out that most of the time PreciseConvexDecomposition was actually far less accurate somehow (when it is meant to be more accurate) ?
Well now i have no idea on how to fix the geometry 
Is it a really complex MeshPart or Union with a lot of concave sections?
Please show all 3 images of how it’s rendered visually, how PreciseComplexDecompostion shows it, and how Default shows it.
Another option is to make the MeshPart or Union CanCollide false, then place Transparent CanCollide Parts in the areas you want items to collide with.