What you see is the expected result. For calculating the bounding box of an element the .getBBox() method is used. The behavior of .getBBox is defined by the SVG spec in 4.5.23 Interface SVGLocatable (emphasis mine):

Returns the tight bounding box in current user space (i.e., after application of the ‘transform’ attribute, if any) on the geometry of all contained graphics elements, exclusive of stroking, clipping, masking and filter effects).

Since all the y-coordinates of your path's move and line-to commands equal 75.00981241717429 the height of the bounding as defined by the spec obviously is zero.

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