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.

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I am using Mathematica 12.0