MATLAB: Union() operation adds hole to polyshape

bugholespolyshapeunion

I have attached a file containing a vector of 11 polyshape objects
>> load('polyshape_vector.mat')
>> whos pgon
  Name      Size            Bytes  Class        Attributes
  pgon      1x11             3080  polyshape
When these polyshapes are plotted, they form an agglomeration of triangles which, to the eye at least, has 2 holes
>> plot(pgon)
However, when I plot the union of this polyshape vector, a 3rd hole forms (this is in R2018a). It defies my intuition that taking the union of a collection of polygons could introduce more holes. Can anyone think why this might occur?
   >> plot(union(pgon))
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