Tracing – Problem with RegionFunction in DensityPlot

I do not understand why RegionFunction connects even places where x + y <1!

{error, xf} = FindGeometricTransform[{{0, 0}, {1, 0}, {1, Tan[Pi/3]} / 2}, {{0, 0}, {1, 0}, {0, 1}}]DP = Show[DensityPlot[x^2*(-1 + x^2/(0.7^2)) + y^2*(-1 + y^2/0.7^2) - y*x, {x, 0, 1}, {y,0, 1}, RegionFunction -> Function[{x, y}, x + y <= 1]]]Graphic[GeometricTransformation[First@DP, xf]]

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As you can see, the points of the triangle respect the state but are missing!