I can plot a BSpline with individual weights:

In[]:=
Graphics@BSplineCurve[{{50,70},{70,50},{50,30},{30,50}},SplineWeights{1,1,1,1},SplineClosedTrue]
Out[]=

Or a filled BSpline with automatic weights:

In[]:=
Graphics@FilledCurve@BSplineCurve[{{50,70},{70,50},{50,30},{30,50}},SplineWeightsAutomatic,SplineClosedTrue]
Out[]=

But a filled BSpline with individual weights doesn’t work:

In[]:=
Graphics@FilledCurve@BSplineCurve[{{50,70},{70,50},{50,30},{30,50}},SplineWeights{1,1,1,1},SplineClosedTrue]
Out[]=