plotting – Wrong boundary in RegionPlot after RegionUnion

I have defined three regions, Slice1, Slice2 and Slice3.

Slice1 = 
  Polygon({{0, 0}, {1, 0}, {1, 1/(Sqrt(3))}, {1/2, Sqrt(3)/2}});
Slice2 = 
  Polygon({{0, 0}, {-1, 0}, {-1, 1/(Sqrt(3))}, {-1/2, Sqrt(3)/2}});
Slice3 = 
  Polygon({{0, 
     0}, {-1/2, -Sqrt(3)/2}, {0, -2/Sqrt(3)}, {1/2, -Sqrt(3)/2}});

CombinedSlice = RegionUnion(Slice1, Slice2, Slice3);
RegionPlot(CombinedSlice, AspectRatio -> Automatic)

When I RegionPlot the combined plot, Slice3 does not have correct boundaries. The boundary shows up correctly when I plot Slice3 individually.

enter image description here

I am using Mathematica 12.0